Alternating phases of FGF receptor and NGF receptor expression in the developing chicken nervous system.

Patterns of expression of transcripts encoding receptors for fibroblast growth factor and nerve growth factor (FGF-R and NGF-R) in the developing chick nervous system are compared using in situ hybridization histochemistry. FGF-R transcripts are expressed abundantly in the germinal neuroepithelial layer. Expression ceases as cells migrate into the mantle layer and returns during late maturation of neuronal populations, including cholinergic nuclei of the basal forebrain, brainstem reticular and motor nuclei, and cerebellar Purkinje and granule neurons. The pattern of NGF-R expression is generally reciprocal to that of FGF-R in the CNS and in some phases of development of the PNS. These results suggest that FGF and NGF may act sequentially rather than in concert during neuronal development.

The DNA sequence specificity of stimulation of DNA polymerases by factor D.

The mechanism of enhancement of DNA polymerase activity by the murine DNA-binding protein factor D was investigated. Extension by Escherichia coli DNA polymerase I and calf thymus DNA polymerase-alpha of 5'-32P-labeled oligodeoxynucleotide primers that are complementary to poly(dT) or to bacteriophage M13 DNA was measured in the absence or presence of factor D. With 5'-[32P](dA)9.poly(dT), factor D enables E. coli polymerase I to fill approximately 15-nucleotide gaps between adjacent primers; whereas in the absence of the stimulatory protein, poly(dT) is not copied significantly. In order to study the nucleotide specificity of synthesis enhancement, we used M13mp10 DNA containing 4 consecutive thymidine residues downstream from the 3-hydroxyl terminus of an oligonucleotide primer. Upon addition of factor D, both polymerase I and polymerase-alpha can traverse this sequence more efficiently and thus generate longer DNA products. Densitometric analysis of nonextended and elongated 5'-32P-labeled M13 primer indicates that, without changing the frequency of primer utilization, factor D enhances the activity of these DNA polymerases by increasing their apparent processivity. By positioning oligonucleotide primers 4, 8, and 12 bases upstream from the (dT)4 template sequence, we show that the enhancement of synthesis by factor D is independent of the position of the oligothymidine cluster. We hypothesize that factor D interacts with oligo(dT).oligo(dA) domains in DNA to alter their conformation, which may normally obstruct the progression of DNA polymerases.