Worm Breeder's Gazette 10(3): 42
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have completed the sequence of unc-86, a gene that controls certain cell lineages (Chalfie et al., Cell 24, 59,1981) and cell differentiations (Finney and Horvitz, WBG 7(2), 45) in C. elegans. We inferred a transcript from the genomic sequence and confirmed some of the splicing patterns with RNase protection experiments. unc-86 encodes a protein with extensive sequence similarity to three mammalian transcription factors: Pit-1, a rat pituitary transcription factor (Ingraham et al., Cell, in press), Oct-1, a ubiquitously- expressed human transcription factor (Strum and Herr, Genes and Development, in press), and Oct-2, a human B cell-specific transcription factor (Clerc et al., Genes and Development, in press). This observation suggests that the unc-86 protein is also a transcription factor. The homologous region, referred to as the pou (pit oct unc, pronounced 'pow') domain, consists of three parts: two 33 amino acid regions, A and B, followed by a 60 amino acid homeodomain (see Figure 1). The homeodomains of the four pou proteins are more similar to each other than they are to other known homeodomains. The four pou homeodomains have several features in common, including similar sequences in the 'helix 3' region (Laughon and Scott, Nature 310, 25, 1984;) (Figure 2). The A and B regions of the pou domain are even more highly conserved among the four proteins than is the homeodomain. The A and B regions and this new class of homeodomain have been found only in these four proteins, always in the order A-B-homeodomain, and with only short sequences between the regions. This similarity among proteins from different animal phyla suggests that the three regions might function as a unit. There is mounting evidence that homeodomains bind DNA; it is possible that the pou domain may act as a functional unit in DNA binding. It seems likely that the pou domain determines the sequence specificity of DNA binding: Oct-1 and Oct-2 are almost completely identical within the pou domain but are different outside it, and they bind to the same DNA sequence. The residues that are conserved in the pou domains of Pit-1 and the Oct proteins probably largely define this specificity, since those proteins have similar binding specificities. These considerations suggest that the unc-86 protein may also be similar in its DNA binding specificity. All of the pou proteins presumably interact with other genes to function. Genetic studies of unc-86 offer an approach to the identification of interacting genes. So far, unc-86 function has been analyzed in two genetic pathways: one that generates the touch receptor neurons and one that generates the HSN neurons. In the touch receptor pathway, unc-86 functions to specify cell lineage and cell fate. unc-86 controls the fate of an ancestor of each touch receptor cell. Acting downstream of unc-86 is the gene mec- 3, which controls the fate of the touch receptor cells and which also contains a homeodomain (Way and Chalfie, Cell 54, 5,1988). It is possible that the unc-86 protein directly regulates mec-3 gene expression. In this case, the mec-3 gene would be expected to contain binding sites for the unc-86 protein. We searched the mec-3 sequence for possible Pit-1 and Oct-1/Oct-2 binding sites. We found a potential Pit-1 binding site within an intron near the probable 5' end of the mec-3 gene and a potential Oct-1/Oct-2 binding site upstream of the probable start of the gene. We do not know if either of these sites is significant. In the HSN pathway unc-86 is necessary for the maturation of that neuron in the L4 stage. Two genes that act upstream of unc-86, control all aspects of HSN phenotype and could be regulators of unc-86 activity in this cell. Two other genes, egl-45 and sem-4, also act at the step of HSN maturation; like unc-86, tropic, and not only controls HSN maturation but also specifies other cell fates (C. Desai, G. Garriga, 5. McIntire, and H.R. Horvitz, manuscript in preparation; M. Stem, personal communication). The egl-45 and sem-4 products might act together with the unc-86 protein to regulate downstream genes. Finally, 16 genes necessary for aspects of HSN differentiation have been identified that act downstream of unc-86; these genes could be transcriptionally regulated by unc-86.In either pathway, it is a reasonable guess that unc-86 may regulate its own expression. We have found a potential Pit-1 binding site in the first intron of unc-86. We do not have enough sequence upstream of the gene to know if there are additional 5' sites. unc-86 was identified genetically, based on its effects on cell fate and cell differentiation. Pit-1, Oct-1, and Oct-2 were identified biochemically as transcription factors. These two very different approaches have identified proteins that share an extensive common domain. Not only does this convergence indicate a biochemical function for some of the genes involved in development, but it also suggests that transcription factors studied biochemically might be involved in the specification of cell type. [See Figure 1 & 2]