lir-2, lir-1 and lin-26 encode a new class of zinc-finger proteins and areorganized in two overlapping operons both in Caenorhabditis elegans and inCaenorhabditis briggsae

lin-26 which encodes a unique Zn-finger protein, is required for differenti ation of nonneuronal ectodermal cells in Caenorhabditis elegans. Here, we s how that the two genes located immediately upstream of lin-26 encode LIN-26 -like Zn-finger proteins; hence their names are lir-1 and lir-2 (lin-26 rel ated). lir-2, lir-1, and lin-26 generate several isoforms by alternative sp licing and/or trans-splicing at different positions. On the basis of their trans-splicing pattern, their intergenic distances, and their expression, w e suggest that lir-2, lir-1, and lin-26 form two overlapping transcriptiona l operons. The first operon, which is expressed in virtually all cells, inc ludes lir-2 and long lir-1 isoforms. The second operon, which is expressed in the nonneuronal ectoderm, includes short lir-1 isoforms, starting at exo n 2 and lin-26. This unusual genomic organization has been conserved in C. briggsae, as shown by cloning the C. briggsae lir-2 lir-1, and lin-26 homol ogs. Particularly striking is the sequence conservation throughout the firs t lir-1 intron, which is very long in both species. Structural conservation is functionally meaningful as C. briggsae lin-26 is also expressed in the nonneuronal ectoderm and can complement a C. elegans lin-26 null mutation.