ORGANIZATION OF THE NEUROFASCIN GENE AND ANALYSIS OF DEVELOPMENTALLY-REGULATED ALTERNATIVE SPLICING

Neurofascin is an axonal member of the L1 subgroup of the immunoglobul in superfamily implicated in neurite extension in the course of embryo nic development. Here we have isolated and characterized the gene enco ding chicken neurofascin. Comparison of genomic sequences with cDNA se quences provides the structure and localization of intron/exon boundar ies and indicates that neurofascin isoforms are generated by alternati ve splicing of its pre-mRNA. The neurofascin gene is composed of 33 ex ons distributed over 72 kilobases. Each of the six immunoglobulin- and five fibronectin-type III-like domains is encoded by two exons. While introns between domains are of phase 1, others are of phase 0, 1, or 2. Alternative splicing of neurofascin is developmentally regulated as shown by polymerase chain reaction analysis. Furthermore, plasmid lib raries from long range polymerase chain reaction-amplified cDNA of neu rofascin were used to examine and quantify the distribution of alterna tively spliced exons in individual neurofascin molecules; We found 50. different neurofascin isoforms at different developmental stages and revealed the existence of one major ''early'' in comparison with multi ple ''late'' neurofascin isoforms.