The gene
lin-26 in C. elegans is required for non-neuronal ectodermal cells to express their normal fates.
lin-26 encodes a presumptive transcription factor with C2H2 motifs related to but different from TFIIIA Zn-fingers. Two genes, named
lir-1 and
lir-2, which encode proteins showing the same type of C2H2 motifs as the LIN-26 protein, are found immediately upstream of
lin-26 (see abstract by Dufourcq and Labouesse). We have cloned and sequenced the homologous genomic region from the related nematode species C. briggsae. The genes
lir-2,
lir-1, and
lin-26, as well as three other genes located upstream of
lir-2, are conserved in C. briggsae and constitute a cluster of genes whose syntenic organisation is conserved. Moreover, the order of the genes, their relative distance, their direction of transcription, the size and position of introns, the patterns of alternative and trans-splicing are extremely well conserved. The LIR-1 and LIN-26 proteins from both species show greater than 75% identity, whereas the LIR-2 proteins are less conserved, showing only 62% identity. Sequence alignments of intergenic regions and of introns allowed us to identify potential regulatory regions that could control expression of the genes
lir-2,
lir-1 and
lin-26. Particularly striking is the sequence conservation in the 9 kb long first
lir-1 intron, an unusually long intron in C. elegans. The nucleotide sequences of the first
lir-1 intron from both species show 75% identity over more than 8 kb (as calculated by the bestfit program from the Wisconsin GCG Package). This intron probably contains promoter elements for
lir-1 and
lin-26. For instance,
lin-26 promoter deletion analysis has shown that a 452 nt-long region is required for normal
lin-26 expression in the somatic gonad (see abstract by Bart den Boer and Michel Labouesse). That small region belongs to a conserved area in the C. briggsae
lir-1 intron 1.