8 GENES AND ALTERNATIVE RNA PROCESSING PATHWAYS GENERATE AN UNEXPECTEDLY LARGE DIVERSITY OF CYTOPLASMIC INTERMEDIATE FILAMENT PROTEINS IN THE NEMATODE CAENORHABDITIS-ELEGANS

Cytoplasmic intermediate filament (IF) proteins of Caenorhabditis elegans are encoded by a dispersed multigene family comprising at least eight genes which map to three linkage groups. Exon sequences and intron patterns define three distinct subfamilies. While all eight IF genes display the long coil 1b subdomain of nuclear lamins, only six genes (a(1)-a(4), b(1) and b(2)) retain a lamin-like tail domain. Two genes (c(1) and c(2)) have acquired entirely novel tail domains. The overall sequence identity of the rod domains is only 29%. The gene structures show a strong drift in number and positions of introns, none of which are common to all genes. Individual genes share only one to four intron locations with the Helix aspersa IF gene, but all eight nematode genes together account for nine of the 10 introns of the gastropod gene. All C.elegans IF genes are transcribed and all except gene c(2) produce trans-spliced mRNAs. Alternatively spliced mRNAs arise from genes a(1), b(2) and c(?)2 through several mechanisms acting at the transcriptional and post-transcriptional levels. These involve the alternative use of distinct promoters, polyadenylation sequences and both cis and trans RNA splice sites. The resulting sequence variations are restricted to the non-helical end domains. Minimally 12 distinct IF proteins are encoded by the various mRNAs. Different abundances in mixed-stage nematode populations suggest cell type- and/or stage-specific expression of individual mRNAs.