ADDITIONAL SEQUENCE COMPLEXITY IN THE MUSCLE GENE, UNC-22, AND ITS ENCODED PROTEIN, TWITCHIN, OF CAENORHABDITIS-ELEGANS

Null mutations of the Caenorhabditis elegans unc-22 gene cause a prono unced body surface twitch associated with impaired movement and disrup tion of muscle structure. Partial sequence analysis of unc-22 has prev iously revealed that its encoded polypeptide, named twitchin, consists of a single protein kinase domain and multiple copies of both an immu noglobulin-like domain and a fibronectin type III-like domain. This pa per reports additional DNA sequence information that has revealed the transcription start of unc-22, the N terminus of twitchin, and an expl anation for the weak phenotype of a transposon insertion allele. These new data indicate that the unc-22 gene is 18 kb larger than previousl y reported and has a transcription unit of 38,308 bp. These data add 7 91 amino acids to the twitchin N terminus for a complete polypeptide s ize of 6,839 amino acids and a predicted molecular weight of 753,494. This new polypeptide sequence includes four additional copies of the a bove-mentioned immunoglobulin-like domains and also includes a glycine -rich sequence that might form a flexible hinge. The additional coding sequence reveals that the insertion of the Tc1 transposon, in the unc -22 allele, st139, should disrupt twitchin structure because it is loc ated in an exon. However, cDNA sequencing has revealed that several cr yptic splice donors and acceptors adjacent to the Tc1 insertion site a re used to splice the transposon out of unc-22(st139) mRNA. One of the se splicing events produces a near wild-type mRNA that deletes only si x amino acids from twitchin, and this might explain the unusually mild phenotype associated with this mutation.