The
unc-13 gene is specifically expressed in many neurons. The product localizes to presynaptic terminals and play a pivotal role in neurotransmission by interacting with oth- er synaptic proteins. The gene is relatively large, 31 kb in length, in the C. elegans genome and contains a large intron, about 9 kp long, which encodes part of another transcript,
unc-13B. The transcript shares its 3'-terminal sequences with the canonical
unc-13A. By comparison between the cDNA and genomic sequence recently determined by the genome sequen- cing consortium, it was uncovered that the largest intron of the
unc-13A has an unusual 5' splice junction sequence, CT, and all the other introns obey the "GT" rule. The same 5' junction also has been observed in an intron after the third "J" segment of the immunoglobulin k light chain gene. Although the
unc-13 sequence should be confirmed by seque- ncing more than one genomes, correlation between the unus- ual intron junction sequence and unusually long intron may suggest that the novel junction sequence regulates the processing of the pre-mRNA and hence gene expression in a neuron-specific manner. Alternatively, inefficient splic- ing through the atypical sequence may be necessary to process correctly the unusually long intron. These working hypotheses will be tested by transplanting the junction to other reporter genes such as beta-galactosidase or green fluorescence protein.