Response of turkey poults to soybean lectin levels typically encountered in commercial diets. 2. Effect on intestinal development and lymphoid organs.

Lectins are capable of altering intestinal morphology by binding to and disrupting the intestinal brush border membrane. They are also known to alter the weight of lymphoid organs. Therefore, we evaluated the effect of soybean lectin (SBL) on intestinal morphology and lymphoid organ weights of poults fed diets containing SBL. Dietary treatments evaluated in this study included a cornstarch and casein-based control (lectin-free) semipurified diet (PD) and semipurified diets containing 0.024 or 0.048% SBL (PDL and PDH, respectively). Experimental diets were fed from hatch to 14 d. Morphological evaluation of the intestine involved measurement of the villi height and perimeter, crypt depth, villus:crypt, and thickness of the muscle layer in the jejunum. Intestinal physical characteristics were also determined by measuring intestinal weight, length, and volume. Results indicated that 0.048% SBL in PDH increased villus:crypt and reduced total intestinal length in turkey poults. In addition, both the 0.024 and 0.048% dietary SBL levels reduced thymus weights. It was concluded that dietary SBL up to 0.048% enhanced intestinal development by increasing villus:crypt, but might alter the structural integrity of lymphoid organs.

Intestinal adaptation to diet in the young domestic and wild turkey (Meleagris gallopavo).

The short-term effects of diet on jejunal growth, alanine transport rate, and leucine aminopeptidase activity (LAP) were compared in the domestic and wild turkey poult. One-day-old poults of each strain were fed diets of high vs., low protein, with carbohydrate varied to maintain isocaloric conditions. Prior to feeding, relative jejunal mass and alanine transport rates were not significantly different in the two turkey strains, whereas LAP activity was 270% higher in wild poults. After feeding for 72 h, relative jejunal mass doubled in both turkey strains. In domestic turkeys, alanine transport rate and LAP activity were reduced by approximately 42% and 25%, respectively, in poults fed a 24% protein-69% carbohydrate diet vs. a 49% protein-35% carbohydrate diet. Analysis of the combined data from feeding experiments revealed that alanine transport rate was not correlated with total food, protein or lipid intake, but was negatively correlated with carbohydrate consumption (P<0.05). In wild turkeys, neither alanine transport rate nor LAP activity were altered by diet. These results reveal that domestic turkey hatchlings can modulate protein digestive and absorptive functions as protein/carbohydrate composition of the diet changes and suggest that high dietary carbohydrate down-regulates the intestinal alanine transporter.

Ecotypic and genetic variation in poplar bark storage protein gene expression and accumulation.

Bark storage proteins (BSP) store nitrogen (N) translocated from senescing leaves in autumn, and supply reduced N for spring growth. Expression of bsp and BSP accumulation are associated with short day photoperiod. To determine if photoperiod-associated bsp expression varies among poplars native to different latitudes, Populus deltoides Bartr. clones originating from six latitudes were grown under natural conditions at a common location. Relative amounts of BSP mRNA in these clones were measured at 2-week intervals from August 7 to October 16. The date of maximum BSP mRNA accumulation was correlated with latitude of origin, and maximum accumulation of BSP mRNA occurred earlier in clones native to northern latitudes than in clones native to southern latitudes. This pattern of variation is consistent with photoperiodic responses of plants native to temperate climates. Genotypic variations in BSP accumulation, bark protein concentration and bark N concentration were compared among clones of six hybrid poplar (Populus trichocarpa Torr. and Gray x P. deltoides) full-sib families (three F(2) families, two F(1) families and one BC(1) family) after 6 weeks in a short day photoperiod and at midwinter. Significant differences in BSP accumulation occurred among clones within four of the six full-sib families after 6 weeks in a short day photoperiod and also at midwinter for outdoor-grown plants. Bark protein and bark N concentrations also varied significantly among clones within certain families. In general, the greatest variation was found in F(2) and BC(1) families. Within several families, relative BSP amounts were positively correlated with bark protein concentration and total bark N concentration. These results indicate a role of photoperiod in regulating bsp expression and demonstrate a genetic component underlying seasonal BSP accumulation. The results could have significance in selecting for clones with improved N storage capacity and N-use efficiency.

TGF-beta inhibits muscle differentiation through functional repression of myogenic transcription factors by Smad3.

Transforming growth factor-beta (TGF-beta) is a potent inhibitor of skeletal muscle differentiation, but the molecular mechanism and signaling events that lead to this inhibition are poorly characterized. Here we show that the TGF-beta intracellular effector Smad3, but not Smad2, mediates the inhibition of myogenic differentiation in MyoD-expressing C3H10T1/2 cells and C2C12 myoblasts by repressing the activity of the MyoD family of transcriptional factors. The Smad3-mediated repression was directed at the E-box sequence motif within muscle gene enhancers and the bHLH region of MyoD, the domain required for its association with E-protein partners such as E12 and E47. The repression could be overcome by supplying an excess of E12, and covalent tethering of E47 to MyoD rendered the E-box-dependent transcriptional activity refractory to the effects of Smad3 and TGF-beta. Smad3 physically interacted with the HLH domain of MyoD, and this interaction correlated with the ability of Smad3 to interfere with MyoD/E protein heterodimerization and binding of MyoD complexes to oligomerized E-box sites. Together, these results reveal a model for how TGF-beta, through Smad3-mediated transcriptional repression, inhibits myogenic differentiation.

ASHP survey of ambulatory care responsibilities of pharmacists in managed care and integrated health systems--2001.

The results of a 2001 national survey of the ambulatory care responsibilities of pharmacists in managed care organizations (MCOs) and integrated health systems are reported and compared with the results of similar surveys conducted in 1997 and 1999. Three hundred and seventy-six MCOs and integrated health systems participated in the telephone survey. The surveyelicited data about organizational structure and pharmacist functions in the ambulatory care environment. Survey respondents were asked about 24 specific ambulatory care pharmacist functions. The performance of functions was related to five "enabling" factors: pharmacists on interdisciplinary care teams, automated dispensing systems, integrated electronic medical records, very supportive medical staff, and very supportive senior management. Twenty previously measured functions decreased since 1999. Decreases were greatest in negotiating pharmaceutical contracts (-28%), administering immunizations (-27%), and immunization screening (-24%). Enabling factors supported continued expansion. Two clusters of functions, patient-related and population-related activities, were identified and supported differentially by enabling factors. Group-model and staff-model HMOs had the most enabling factors and the broadest scope of pharmacist functions. Independent practice associations had fewer enabling factors and a different mix of functions, with an emphasis on population-focused functions. Ambulatory care functions of pharmacists have expanded to new areas and have decreased in more traditional areas, perhaps because of the current pharmacist shortage and the increase in the number of prescriptions and patients.

Activated notch inhibits myogenic activity of the MADS-Box transcription factor myocyte enhancer factor 2C.

Skeletal muscle gene expression is dependent on combinatorial associations between members of the MyoD family of basic helix-loop-helix (bHLH) transcription factors and the myocyte enhancer factor 2 (MEF2) family of MADS-box transcription factors. The transmembrane receptor Notch interferes with the muscle-inducing activity of myogenic bHLH proteins, and it has been suggested that this inhibitory activity of Notch is directed at an essential cofactor that recognizes the DNA binding domains of the myogenic bHLH proteins. Given that MEF2 proteins interact with the DNA binding domains of myogenic bHLH factors to cooperatively regulate myogenesis, we investigated whether members of the MEF2 family might serve as targets for the inhibitory effects of Notch on myogenesis. We show that a constitutively activated form of Notch specifically blocks DNA binding by MEF2C, as well as its ability to cooperate with MyoD and myogenin to activate myogenesis. Responsiveness to Notch requires a 12-amino-acid region of MEF2C immediately adjacent to the DNA binding domain that is unique to this MEF2 isoform. Two-hybrid assays and coimmunoprecipitations show that this region of MEF2C interacts directly with the ankyrin repeat region of Notch. These findings reveal a novel mechanism for Notch-mediated inhibition of myogenesis and demonstrate that the Notch signaling pathway can discriminate between different members of the MEF2 family.

Differential effects of fat and sucrose on body composition in A/J and C57BL/6 mice.

The C57BL/6 (B6) mouse is more sensitive to the effects of a high-fat diet than the A/J strain. The B6 mouse develops severe obesity, hyperglycemia, and hyperinsulinemia when fed this dietary regimen. This study was conducted to determine the effects of dietary fat and sucrose concentrations on body composition and intestinal sucrase (EC 3.2.1.48) and maltase (EC 3.2.1.20) activity in these two mouse strains. High-fat diets, regardless of sucrose content, resulted in significant weight gain, higher body fat, and lower body protein and water content in both strains of mice. The shift toward higher body fat and lower protein and water content was far greater in the B6 strain. Low-fat, high-sucrose diets resulted in lower body weight in both strains, as well as significantly greater body protein content in B6 mice. Analysis of intestinal sucrase showed that the enzyme was less active in B6 mice when the diet was high in sucrose. Both sucrase and maltase had lower activity in the presence of high dietary fat in both mouse strains. The percent reduction of intestinal enzyme activity due to dietary fat was similar in both strains. The B6 mouse exhibits disproportionate weight gain and altered body composition on a high-fat diet. This coupled with the reduced body weight and increased body protein on a low-fat, high-sucrose diet suggests that factors-relative to fat metabolism rather than sucrose metabolism are responsible for obesity.

The myogenic regulatory gene Mef2 is a direct target for transcriptional activation by Twist during Drosophila myogenesis.

MEF2 is a MADS-box transcription factor required for muscle development in Drosophila. Here, we show that the bHLH transcription factor Twist directly regulates Mef2 expression in adult somatic muscle precursor cells via a 175-bp enhancer located 2245 bp upstream of the transcriptional start site. Within this element, a single evolutionarily conserved E box is essential for enhancer activity. Twist protein can bind to this E box to activate Mef2 transcription, and ectopic expression of twist results in ectopic activation of the wild-type 175-bp enhancer. By use of a temperature-sensitive mutant of twist, we show that activation of Mef2 transcription via this enhancer by Twist is required for normal adult muscle development, and reduction in Twist function results in phenotypes similar to those observed previously in Mef2 mutant adults. The 175-bp enhancer is also active in the embryonic mesoderm, indicating that this enhancer functions at multiple times during development, and its function is dependent on the same conserved E box. In embryos, a reduction in Twist function also strongly reduced Mef2 expression. These findings define a novel transcriptional pathway required for skeletal muscle development and identify Twist as an essential and direct regulator of Mef2 expression in the somatic mesoderm.

Multiple roles for the MyoD basic region in transmission of transcriptional activation signals and interaction with MEF2.

Establishment of skeletal muscle lineages is controlled by the MyoD family of basic helix-loop-helix (bHLH) transcription factors. The ability of these factors to initiate myogenesis is dependent on two conserved amino acid residues, alanine and threonine, in the basic domains of these factors. It has been postulated that these two residues may be responsible for the initiation of myogenesis via interaction with an essential myogenic cofactor. The myogenic bHLH proteins cooperatively activate transcription and myogenesis through protein-protein interactions with members of the myocyte enhancer factor 2 (MEF2) family of MADS domain transcription factors. MyoD-E12 heterodimers interact with MEF2 proteins to synergistically activate myogenesis, while homodimers of E12, which lack the conserved alanine and threonine residues in the basic domain, do not interact with MEF2. We have examined whether the myogenic alanine and threonine in the MyoD basic region are required for interaction with MEF2. Here, we show that substitution of the MyoD basic domain with that of E12 does not prevent interaction with MEF2. Instead, the inability of alanine-threonine mutants of MyoD to initiate myogenesis is due to a failure to transmit transcriptional activation signals provided either from the MyoD or the MEF2 activation domain. This defect in transcriptional transmission can be overcome by substitution of the MyoD or the MEF2 activation domain with the VP16 activation domain. These results demonstrate that myogenic bHLH-MEF2 interaction can be uncoupled from transcriptional activation and support the idea that the myogenic residues in myogenic bHLH proteins are essential for transmission of a transcriptional activation signal.

Transcriptional control of muscle development by myocyte enhancer factor-2 (MEF2) proteins.

Metazoans contain multiple types of muscle cells that share several common properties, including contractility, excitability, and expression of overlapping sets of muscle structural genes that mediate these functions. Recent biochemical and genetic studies have demonstrated that members of the myocyte enhancer factor-2 (MEF2) family of MADS (MCM1, agamous, deficiens, serum response factor)-box transcription factors play multiple roles in muscle cells to control myogenesis and morphogenesis. Like other MADS-box proteins, MEF2 proteins act combinatorially through protein-protein interactions with other transcription factors to control specific sets of target genes. Genetic studies in Drosophila have also begun to reveal the upstream elements of myogenic regulatory hierarchies that control MEF2 expression during development of skeletal, cardiac, and visceral muscle lineages. Paradoxically, MEF2 factors also regulate cell proliferation by functioning as endpoints for a variety of growth factor-regulated intracellular signaling pathways that are antagonistic to muscle differentiation. We discuss the diverse functions of this family of transcription factors, the ways in which they are regulated, and their mechanisms of action.

The MEF2A 3' untranslated region functions as a cis-acting translational repressor.

Myocyte enhancer factor 2 (MEF2) proteins serve as important muscle transcription factors. In addition, MEF2 proteins have been shown to potentiate the activity of other cell-type-specific transcription factors found in muscle and brain tissue. While transcripts for MEF2 factors are widely expressed in a variety of cells and tissues, MEF2 proteins and binding activity are largely restricted to skeletal, smooth, and cardiac muscle and to brain. This disparity between MEF2 protein and mRNA expression suggests that translational control may play an important role in regulating MEF2 expression. In an effort to identify sequences within the MEF2A message which control translation, we isolated the mouse MEF2A 3' untranslated region (UTR) and fused it to the chloramphenicol acetyltransferase (CAT) reporter gene. Here, we show by CAT assay that the MEF2A 3' UTR dramatically inhibits CAT gene expression in vivo and that this inhibition is due to an internal region within the highly conserved 3' UTR. RNase protection analyses demonstrated that the steady-state level of CAT mRNA produced in vivo was not affected by fusion of the MEF2A 3' UTR, indicating that the inhibition of CAT activity resulted from translational repression. Furthermore, fusion of the MEF2A 3' UTR to CAT inhibited translation in vitro in rabbit reticulocyte lysates. We also show that the translational repression mediated by the 3' UTR of MEF2A is regulated during muscle cell differentiation. As muscle cells in culture differentiate, the translational inhibition caused by the MEF2A 3' UTR is relaxed. These results demonstrate that the MEF2A 3' UTR functions as a cis-acting translational repressor both in vitro and in vivo and suggest that this repression may contribute to the tissue-restricted expression and binding activity of MEF2A.

Jejunal glucose uptake and oxygen consumption in turkey poults selected for rapid growth.

Two lines of turkey poults, one selected for rapid growth at 16 wk of age (F line) and the other a randombred control line (RBC2) were used to investigate the effect of selection for rapid growth on jejunal O2 consumption and glucose transport as well as whole-body O2 consumption. All trials used unsexed poults and were designed as a randomized complete block with day and line as independent variables. In Trial 1, 120 turkey poults, fed a standard starter ration (25.5% CP), were used to examine the effect of selection on feed intake, body weight gain, and efficiency from hatching (Day 0) to 13 d of age. At Day 14, 36 of 60 birds from each line were killed to measure intestinal length and weight and jejunal O2 consumption after 18 h of feed deprivation. Compared with the RBC2 line, the F line had relatively shorter but heavier small intestinal segments when adjusted by 18 h feed-deprived body weight (FBW; P < 0.001). The F line consumed more O2 over the entire jejunum adjusted to FBW than RBC2 line (43.8 vs 34.6 nmol O2/min.g FBW; P < 0.001). Jejunal ouabain- and cycloheximide-sensitive O2 consumption were greater (P < 0.05) in the F line. In Trial 2, 16 14-d-old poults from each line were used to measure in vitro jejunal glucose transport rate. There was no difference in glucose transport of the jejunum (nanomoles per minute per gram of FBW) between the lines. In Trial 3, 20 poults from each line were used to measure whole-body O2 consumption at 7 to 10 d of age. The F and RBC2 lines had similar whole-body O2 consumption rate per gram of FBW. These data suggest that selection of turkeys for rapid growth at 16 wk of age did not increase efficiency of jejunal glucose uptake in 14-d-old turkey poults.

Temporal artery tap: usefulness and limitations in carotid sonography.

PURPOSE: To examine the effectiveness of percussion of the superficial temporal artery for identification of the external carotid artery (ECA). MATERIALS AND METHODS: The temporal artery tap maneuver was performed on 324 carotid arteries (163 patients). Evidence for transmission of the effect of the temporal tap was sought in the pulsed Doppler ultrasound waveforms of the ECA, common carotid artery (CCA), and internal carotid artery (ICA). The location and severity of stenotic lesions were recorded. The relative amplitudes of the oscillations created by the tap were compared. RESULTS: The temporal tap effect could be seen in 262 ECAs (81%), 174 CCAs (54%), and 106 ICAs (33%). The tap effect can be seen in the ICA at all grades of ICA disease. When the oscillations were seen in only one of the two major branches, that branch was always the ECA. When the temporal tap effect was found in the ICA, the amplitudes of the oscillations were the same as or greater than those of the ECA in 26% of cases. CONCLUSION: Waveform oscillations from the temporal tap maneuver often can be found beyond the ECA in the CCA and ICA. Thus, the temporal tap alone may not reliably distinguish the ECA from the ICA or CCA.

Selection for growth does not affect apparent energetic efficiency of jejunal glucose uptake in mice.

Five-wk-old male mice from high growth (M16) and randomly bred control (ICR) lines, plus their reciprocal crosses, ICR x M16 and M16 x ICR, were used to investigate whether whole-body O2 consumption, jejunal respiration, jejunal glucose absorption and the apparent energetic efficiency of jejunal active glucose uptake in mice are altered by genetic selection for growth as well as by heterosis and maternal effects. Whole-body O2 consumption was measured in 12 mice from each line or cross. The mice were later killed for measurement of jejunal O2, using tissue respiration chambers and jejunal glucose transport determined by 3H-3-O-methylglucose accumulation. No heterosis or maternal effects were detected in jejunal glucose active transport and active glucose uptake. Selection for growth (M16 vs. ICR) increased daily gain (1.54 vs. 1.09 g, P < 0.001), small intestinal length and weight, but did not enhance jejunal glucose transport. The apparent energetic efficiency of jejunal active glucose uptake among lines was not different (54.0, 50.4, 51.6 and 47.1 nmol ATP expended/nmol glucose uptake for M16, ICR, M16 x ICR and ICR x M16, respectively, P > 0.63). Selection for growth in mice did not result in more energetically efficient jejunal glucose absorption.

Selection for body composition does not affect energetic efficiency of jejunal glucose uptake in mice.

Five-wk-old male mice from three lines were used to examine whether the apparent energetic efficiency of active jejunal glucose uptake in mouse jejunum is altered by genetic selection for different body composition. The mice lines were selected as follows: HE, high percentage of body fat with no change in body weight as a constraint; LF, low percentage of body fat; and RS, randomly bred control. Body weight was similar in all lines. Total jejunal O2 consumption and ouabain-sensitive O2 consumption were used to estimate the energy expenditure associated with glucose absorption and Na+/K(+)-ATPase activity. Tritiated 3-O-methyl-D-glucose was used to determine glucose uptake by mouse jejunum. Line LF, when compared with line HE, had lower body fat as indicated by epididymal fat pad weight (143 vs. 362 mg/mouse, P < 0.001). There were no significant differences in small intestinal weight, length and density (mg/cm) between LF and HE lines. Jejunal villus width was greater in line LF compared with line HE (115 vs. 92 microm, P < 0.002). Jejunal glucose transport and O2 consumption were not different between LF and HE lines. Ouabain-sensitive O2 consumption was not significantly different among the three lines. No differences were noted in the apparent energetic efficiency of active glucose uptake among lines.

Peptide regulation of intestinal glucose absorption.

Terminal hydrolysis of oligosaccharides at the small intestinal brush border yields monomeric glucose, most of which is then absorbed by the transepithelial route. This involves carrier-mediated processes requiring specialized functional proteins situated in the brush border (SGLT1) and basolateral (GLUT2) membranes. Glucose translocation at the enterocyte apical membrane is an active, Na(+)-dependent and saturable process, whereas exit from enterocytes is by facilitated diffusion and is energy-independent. Specific adaptation of glucose active transport occurs in response to changes in the proportion of glucose in the diet. The regulatory signals responsible for transport induction are imprecisely defined, although numerous protein hormones and gut regulatory proteins are implicated. Epidermal growth factor and peptide YY invoke up-regulation of jejunal active glucose transport in vivo. Recently, peptide YY has been shown to stimulate active glucose transport in mice without altering oxygen consumption of jejunal tissue. Several other peptides whose presence in tissues of the small bowel imply that they exert control over epithelial nutrient transport are considered, and the relevance of these physiological manipulations, with various regulatory peptides and hormones, to animal agriculture are discussed.

Cooperative transcriptional activation by the neurogenic basic helix-loop-helix protein MASH1 and members of the myocyte enhancer factor-2 (MEF2) family.

Establishment of skeletal muscle and neural cell types is controlled by families of myogenic and neurogenic basic helix-loop-helix (bHLH) proteins, respectively. Myogenic bHLH proteins have been shown to activate skeletal muscle transcription in collaboration with members of the myocyte enhancer factor-2 (MEF2) family of MCM1-agamous-deficiens-serum response factor (MADS)-box transcription factors, which are expressed in differentiated myocytes and neurons. Here, we show that the neurogenic bHLH protein MASH1 interacts with members of the MEF2 family and that this interaction, mediated by the DNA binding and dimerization domains of these factors, results in synergistic activation of transcription through either the MASH1 or the MEF2 DNA binding site. Consistent with their involvement in activation of neuronal gene expression, members of the MEF2 family are expressed in P19 embryonal carcinoma cells that have been induced to form neurons following treatment with retinoic acid. These results suggest that members of the MEF2 family perform similar roles in synergistic activation of transcription in myogenic and neurogenic lineages by serving as cofactors for cell type-specific bHLH proteins.

MEF2B is a potent transactivator expressed in early myogenic lineages.

There are four members of the myocyte enhancer binding factor 2 (MEF2) family of transcription factors, MEF2A, -B, -C, and -D, that have homology within an amino-terminal MADS box and an adjacent MEF2 domain that together mediate dimerization and DNA binding. MEF2A, -C, and -D have previously been shown to bind an A/T-rich DNA sequence in the control regions of numerous muscle-specific genes, whereas MEF2B was reported to be unable to bind this sequence unless the carboxyl terminus was deleted. To further define the functions of MEF2B, we analyzed its DNA binding and transcriptional activities. In contrast to previous studies, our results show that MEF2B binds the same DNA sequence as other members of the MEF2 family and acts as a strong transactivator through that sequence. Transcriptional activation by MEF2B is dependent on the carboxyl terminus, which contains two conserved sequence motifs found in all vertebrate MEF2 factors. During mouse embryogenesis, MEF2B transcripts are expressed in the developing cardiac and skeletal muscle lineages in a temporospatial pattern distinct from but overlapping with those of the other Mef2 genes. The mouse Mef2b gene maps to chromosome 8 and is unlinked to other Mef2 genes; its intron-exon organization is similar to that of the other vertebrate Mef2 genes and the single Drosophila Mef2 gene, consistent with the notion that these different Mef2 genes evolved from a common ancestral gene.

The effect of hydrocortisone and thyroxine on development of calcium homeostasis in embryonic intestinal epithelium.

Cytoplasmic Ca2+ concentration of epithelial cells from 14-day embryonic chick duodena decreased during 72 h of organ culture to a value 54% of that found at 17 days in vivo. The ability of cells to maintain a constant Ca2+ concentration when challenged with high extracellular calcium was also significantly reduced. Addition of 1 microM hydrocortisone during culture restored both parameters of Ca2+ homeostasis to that of 16-day uncultured duodena, and rise in cytoplasmic Ca2+ was significant within 4 h of hormone treatment. Thyroxine influenced epithelial Ca2+ similarly, but to a lesser degree and only after 48-72 h of culture. These data indicate that glucocorticoids, and possibly thyroid hormones, influence the development of calcium homeostasis in intestinal epithelium.

Mutational analysis of the DNA binding, dimerization, and transcriptional activation domains of MEF2C.

There are four members of the myocyte enhancer factor 2 (MEF2) family of transcription factors in vertebrates, MEF2A, -B, -C, and -D, which have homology within a MADS box at their amino termini and an adjacent motif known as the MEF2 domain. These factors activate muscle gene expression by binding as homo- and heterodimers to an A/T-rich DNA sequence in the control regions of muscle-specific genes. To understand the mechanisms of muscle gene activation of MEF2 factors, we generated a series of deletion and site-directed mutants of MEF2C. These mutants demonstrated that the MADS and MEF2 domains mediate DNA binding and dimerization, whereas the carboxyl terminus is required for transcriptional activation. Amino acids that are essential for MEF2 site-dependent transcription but which do not affect DNA binding were also identified in the MEF2 domain. This type of positive-control mutant demonstrates that the transcription activation domain of MEF2C, although separate from the MEF2 domain, is dependent on this domain for transcriptional activation through the MEF2 site. MEF2 mutants that are defective for DNA binding act as dominant negative mutants and can inhibit activation of MEF2-dependent genes by wild-type MEF2C.