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lin-26, which encodes a unique Zn-finger protein, is required for differentiation of nonneuronal ectodermal cells in Caenorhabditis elegans. Here, we show 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 related). lir-2, lir-1, and lin-26 generate several isoforms by alternative splicing and/or trans-splicing at different positions. On the basis of their trans-splicing pattern, their intergenic distances, and their expression, we suggest that lir-2, lir-1, and lin-26 form two overlapping transcriptional operons. The first operon, which is expressed in virtually all cells, includes lir-2 and long lir-1 isoforms. The second operon, which is expressed in the nonneuronal ectoderm, includes short lir-1 isoforms, starting at exon 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 homologs. Particularly striking is the sequence conservation throughout the first 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.

APETALA2 (AP2) plays a central role in the establishment of the floral meristem, the specification of floral organ identity, and the regulation of floral homeotic gene expression in Arabidopsis. We show here that in addition to its functions during flower development, AP2 activity is also required during seed development. We isolated the AP2 gene and found that it encodes a putative nuclear protein that is distinguished by an essential 68-amino acid repeated motif, the AP2 domain. Consistent with its genetic functions, we determined that AP2 is expressed at the RNA level in all four types of floral organs--sepals, petals, stamens, and carpels--and in developing ovules. Thus, AP2 gene transcription does not appear to be spatially restricted by the floral homeotic gene AGAMOUS as predicted by previous studies. We also found that AP2 is expressed at the RNA level in the inflorescence meristem and in nonfloral organs, including leaf and stem. Taken together, our results suggest that AP2 represents a new class of plant regulatory proteins that may play a general role in the control of Arabidopsis development.