So far, 4 classes of mutation have been identified at the
cha-1- unc- 17 complex locus:
cha-1 and
unc-17 mutations complement each other, while the a and J3 classes fail to complement both
cha-1 and
unc-17 mutations.
cha-1 and a mutants are deficient in choline acetyltransferase (ChAT) activity, B mutants are partially deficient, and
unc-17 animals have normal ChAT activity. Fine-structure genetic mapping data indicates that
cha-1 mutants map to the left of
unc-17, with the anomalous a allele between them and the alleles to the right of
unc-17 (Rand, 1 989). We obtained Tc1
cha-1 alleles, and used them to clone part of the region; we have now cloned and analyzed more than 14 kb of genomic DNA, as well as cDNAs obtained from the Kim and Barstead libraries. We have identified and characterized two classes of cDNA which appear to arise through alternative splicing and which correspond to
cha-1 and
unc-17 transcripts, respectively. Sequence analysis of
cha-1 mutants confirms the identity of the
cha-1 transcript; sequence comparison with ChAT from other species confirms that
cha-1 encodes ChAT (see abstract by Alfonso, et al.). Transcription of both transcripts is from right to left, and both share a common 5' untranslated exon at the right end of the region. The
cha-1 transcript then has a very long (~5kb) intron. The
unc-17 transcript has a much shorter intron, and the entire
unc-17 transcript lies within the long
cha-1 intron. These results suggest an explanation of the genetic data. The J3 mutations presumably are in the common 5' exon or near it, and we have shown that the a mutation is a deletion removing the 3' end of the putative
unc-17 transcript and affecting sequences upstream of the splice acceptor site at the end of the long
cha-1 intron. The nature and function of the
unc-17-encoded protein is unclear. Supported by grants from NIGMS and NSF.