The achaete-scute proteins from Drosophila are a family of basic/helix-loop-helix (bHLH) proteins required for neuronal cell fate specification. We have isolated a new basic/helix-loop-helix (bHLH) protein in C. elegans, called
hlh-3 (MW=~20,700), by exploiting the potential for bHLH proteins to form dimers in solution. This was accomplished by probing an embryonic cDNA expression library (courtesy of A. Fire and P. Okkema) with radiolabelled human E12 bHLH protein. Four other achaete-scute like bHLH genes have been found, three from the Genome Sequencing project (T05G5.2, C34E10.7, and T15H9.3), and one recently by Scott Emmons (
lin-32 ).
hlh-3 has a highly unusual intron/exon structure for a bHLH protein. Generally, the bHLH domain resides completely on one exon; however, in the case of
hlh-3, a small 450 bp intron splits the bHLH domain. None of the bHLH genes in other organisms have this intron/exon structure. Interestingly, this split bHLH domain is shared by the four other achaete-scute-like genes (T05G5.2, C34E10.7, T15H9.3 and
lin-32) but not by the non-achaete-scute -like bHLH genes (
hlh-1,
hlh-2, and F58A4.7). Furthermore, we have found that both
hlh-3 and T05G5.2 are members of an operon. We speculate that the tissue specific expression of these five genes, especially those within an operon, maybe regulated at the level of splicing and are testing this hypothesis by studying the ability of the intron to direct gene expression. Finally, to fully understand the role of these genes (
hlh-3, T05G5.2, C34E10.7 and T15H9.3) in the worm, we have used the method of targeted gene disruption to create a putative null allele. We have already identified a Tc1 insertion in
hlh-3 (we thank Cori Bargmann for access to her library) and are currently screening for a disruption of this gene. We are also screening for insertions in the other three genes.