In C. elegans the gene
lin-26 is required to specify and/or maintain the fates of hypodermal and support cells.
lin-26 encodes a putative transcription factor with two C2H2 zinc-finger motifs that are related to but different from the TFIIIA zinc-finger motifs. The two genes located immediately upstream of
lin-26 are predicted to code for the same atypical C2H2 zinc finger motifs. Together with
lin-26 these genes define a new C2H2 motif, hence their names
lir-1 and
lir-2 for
lin-26 related genes (the order of the three genes is
lir-2/lir-1/lin-26). In order to characterise
lir-1 and
lir-2 we first analyzed their genomic organization and their expression pattern. 	
lir-2,
lir-1 and
lin-26 can generate multiple isoforms through trans-splicing at different exons and/or alternative splicing. Several lines of evidence suggest that
lir-2,
lir-1 and
lin-26 are organized in a bipartite operon comprising two transcription units. The upstream unit would include
lir-2 and long
lir-1 isoforms starting at exon 1; the second unit would include short
lir-1 isoforms starting at exon 2 and
lin-26. The first and second
lir-1 exons are separated by a 9 kb intron, an unusually long intron in C. elegans. Data in favor of this model are provided by the analysis of the trans-splicing and expression patterns of these genes. First, we found that
lir-2 transcripts are trans-spliced by SL1 and that long
lir-1 isoforms are trans-spliced by SL4. Second, we found that short
lir-1 isoforms are trans-spliced by SL1 and that
lin-26 isoforms are trans-spliced by SL2. Third, in situ hybridization and lacZ reporter constructs showed that
lir-2 and
lir-1 are ubiquitously expressed in early embryos until the 200-cell stage, that
lir-2 and the long
lir-1 isoforms are expressed in all neurons of the animal during larval development, that
lir-1 and
lin-26 are expressed during embryogenesis in all hypodermal and glial-like cells and during larval development in all minor hypodermal cells (i.e. all hypodermal cells but major hypodermal cells of the body) and all support cells. During larval development,
lir-1 is uniquely expressed in the pharynx and
lin-26 in major hypodermal cells of the body. The genomic organization of these three genes (including the position, the length and the sequence of the long
lir-1 intron) has been conserved in the nematode C. briggsae. 	To determine if
lir-2 and
lir-1 act in the same pathway as
lin-26, we tried to isolate mutants and also made double stranded RNA injection. While
lir-2(RNAi) animals appeared to be wild-type,
lir-1(RNAi) embryos failed to hatch. We have recently isolated a 3 kb deletion within
lir-1 mutant after PCR-screening pools of TMP/UV mutagenized animals. Further information will be reported at the meeting.