Genetic and behavioral differences among five inbred mouse strains commonly used in the production of transgenic and knockout mice.

Five strains of mice commonly used in transgenic and knockout production were compared with regard to genetic background and behavior. These strains were: C57BL/6J, C57BL/6NTac, 129P3/J (formerly 129/J), 129S6/SvEvTac (formerly 129/SvEvTac) and FVB/NTac. Genotypes for 342 microsatellite markers and performance in three behavioral tests (rotorod, open field activity and habituation, and contextual and cued fear conditioning) were determined. C57BL/6J and C57BL/6NTac were found to be true substrains; there were only 12 microsatellite differences between them. Given the data on the genetic background, one might predict that the two C57BL/6 substrains should be very similar behaviorally. Indeed, there were no significant behavioral differences between C57BL/6J and C57BL/6NTac. Contrary to literature reports on other 129 strains, 129S6/SvEvTac often performed similarly to C57BL/6 strains, except that it was less active. FVB/NTac showed impaired rotorod learning and cued fear conditioning. Therefore, both 129S6/SvEvTac and C57BL/6 are recommended as background strains for targeted mutations when researchers want to evaluate their mice in any of these three behavior tests. However, any transgene on the FVB/NTac background should be transferred to B6. Habituation to the open field was analyzed using the parameters: total distance, center distance, velocity and vertical activity. Contrary to earlier studies, we found that all strains habituated to the open field in at least two of these parameters (center distance and velocity).

The complete DNA sequence of the Ectocarpus siliculosus Virus EsV-1 genome.

The Ectocarpus siliculosus Virus-1, EsV-1, is the type-species of a genus of Phycodnaviridae, the phaeoviruses, infecting marine filamentous brown algae. The EsV-1 genome of 335,593 bp contains tandem and dispersed repetitive elements in addition to a large number of open reading frames of which 231 are currently counted as genes. Many genes can be assigned to functional groups involved in DNA synthesis, DNA integration, transposition, and polysaccharide metabolism. Furthermore, EsV-1 contains components of a surprisingly complex signal transduction system with six different hybrid histidine protein kinases and four putative serine/threonine protein kinases. Several other genes encode polypeptides with protein-protein interaction domains. However, 50% of the predicted genes have no counterparts in data banks. Only 28 of the 231 identified genes have significant sequence similarities to genes of the Chlorella virus PBCV-1, another phycodnavirus. To our knowledge, the EsV-1 genome is the largest viral DNA sequenced to date.

Analysis of the chromosome sequence of the legume symbiont Sinorhizobium meliloti strain 1021.

Sinorhizobium meliloti is an alpha-proteobacterium that forms agronomically important N(2)-fixing root nodules in legumes. We report here the complete sequence of the largest constituent of its genome, a 62.7% GC-rich 3,654,135-bp circular chromosome. Annotation allowed assignment of a function to 59% of the 3,341 predicted protein-coding ORFs, the rest exhibiting partial, weak, or no similarity with any known sequence. Unexpectedly, the level of reiteration within this replicon is low, with only two genes duplicated with more than 90% nucleotide sequence identity, transposon elements accounting for 2.2% of the sequence, and a few hundred short repeated palindromic motifs (RIME1, RIME2, and C) widespread over the chromosome. Three regions with a significantly lower GC content are most likely of external origin. Detailed annotation revealed that this replicon contains all housekeeping genes except two essential genes that are located on pSymB. Amino acid/peptide transport and degradation and sugar metabolism appear as two major features of the S. meliloti chromosome. The presence in this replicon of a large number of nucleotide cyclases with a peculiar structure, as well as of genes homologous to virulence determinants of animal and plant pathogens, opens perspectives in the study of this bacterium both as a free-living soil microorganism and as a plant symbiont.

The composite genome of the legume symbiont Sinorhizobium meliloti.

The scarcity of usable nitrogen frequently limits plant growth. A tight metabolic association with rhizobial bacteria allows legumes to obtain nitrogen compounds by bacterial reduction of dinitrogen (N2) to ammonium (NH4+). We present here the annotated DNA sequence of the alpha-proteobacterium Sinorhizobium meliloti, the symbiont of alfalfa. The tripartite 6.7-megabase (Mb) genome comprises a 3.65-Mb chromosome, and 1.35-Mb pSymA and 1.68-Mb pSymB megaplasmids. Genome sequence analysis indicates that all three elements contribute, in varying degrees, to symbiosis and reveals how this genome may have emerged during evolution. The genome sequence will be useful in understanding the dynamics of interkingdom associations and of life in soil environments.

Progress in Arabidopsis genome sequencing and functional genomics.

Arabidopsis thaliana has a relatively small genome of approximately 130 Mb containing about 10% repetitive DNA. Genome sequencing studies reveal a gene-rich genome, predicted to contain approximately 25000 genes spaced on average every 4.5 kb. Between 10 to 20% of the predicted genes occur as clusters of related genes, indicating that local sequence duplication and subsequent divergence generates a significant proportion of gene families. In addition to gene families, repetitive sequences comprise individual and small clusters of two to three retroelements and other classes of smaller repeats. The clustering of highly repetitive elements is a striking feature of the A. thaliana genome emerging from sequence and other analyses.

Cell proliferation in a peripheral target is required for the induction of central neurogenesis in the leech.

Several days after the completion of the early phase of cell proliferation that generates most of the leech central nervous system, the pair of "sex ganglia" in the two reproductive segments of the midbody undergo a second period of neurogenesis that gives rise to several hundred peripherally induced central (PIC) neurons. This proliferative phase, which begins on embryonic day 17 (E17), is induced by the interaction of a few specific neurons in the sex ganglia with a peripheral target, the male genitalia, during a critical period that extends from E13 to E16. The central nervous system (CNS) determines the critical period, since the male genitalia have the capacity to induce PIC neurons beginning on E10 and continuing throughout embryogenesis. Here we first show, by injecting hydroxyurea into staged embryos to ablate dividing cells, that PIC neuron precursors begin to divide at a low rate before E17, during the critical period. Then, through a series of homochronic and heterochronic male organ transplantations combined with hydroxyurea treatment of hosts and/or donors, we show that cell proliferation is required in the target itself for it to be competent to induce PIC neurons. These observations demonstrate that a nerve connection can couple cell proliferation in a peripheral target to cell proliferation in the CNS, providing a novel means for size adjustment of a central neuronal population relative to a peripheral target.

Identified central neurons convey a mitogenic signal from a peripheral target to the CNS.

Regulation of central neurogenesis by a peripheral target has been previously demonstrated in the ventral nerve cord of the leech Hirudo medicinalis (Baptista, C. A., Gershon, T. R. and Macagno, E. R. (1990). Nature 346, 855-858) Specifically, innervation of the male genitalia by the fifth and sixth segmental ganglia (the sex ganglia) was shown to trigger the birth of several hundred central neurons (PIC neurons) in these ganglia. As reported here, removal of the target early during induction shows that PIC neurons can be independently induced in each side of a ganglion, indicating that the inductive signal is both highly localized and conveyed to each hemiganglion independently. Further, since recent observations (Becker, T., Berliner, A. J., Nitabach, M. N., Gan, W.-B. and Macagno, E. R. (1995). Development, 121, 359-369) had indicated that efferent projections are probably involved in this phenomenon, we individually ablated all possible candidates, which led to the identification of two central neurons that appear to play significant roles in conveying the inductive signal to the CNS. Ablation of a single ML neuron reduced cell proliferation in its own hemiganglion by nearly 50%, on the average. In contrast, proliferation on the opposite side of the ganglion increased by about 25%, suggesting the possibility of a compensatory response by the remaining contralateral ML neuron. Simultaneous ablation of both ML neurons in a sex ganglion caused similar reductions in cell proliferation in each hemiganglion. Deletion of a single AL neuron produced a weaker (7%) but nonetheless reproducible reduction. Ablation of the other nine central neurons that might have been involved in PIC neuron induction had no detectable effect. Both ML and AL neurons exhibit ipsilateral peripheral projections, and both arborize mostly in the hemiganglion where they reside. Thus, we conclude that peripheral regulation of central neurogenesis is mediated in the leech by inductive signals conveyed retrogradely to each hemiganglion by specific central neurons that innervate this target and the hemiganglion they affect.