In the early Caenorhabditis elegans embryo, maternally expressed PIE-1 protein is required in germ-line blastomeres to inhibit somatic differentiation, maintain an absence of mRNA transcription, and block phosphorylation of the RNA polymerase II large subunit (Pol II) carboxy-terminal domain (CTD). We have determined that PIE-1 can function as a transcriptional repressor in cell culture assays. By fusing PIE-1 sequences to the yeast GAL4 DNA-binding domain, we have identified a PIE-1 repression domain that appears to inhibit the transcriptional machinery directly. A sequence element that is required for this repressor activity is similar to the Pol II CTD heptapeptide repeat, suggesting that the PIE-1 repression domain might target a protein complex that can bind the CTD. An alteration of this sequence element that blocks repression also impairs the ability of a transgene to rescue a pie-1 mutation, suggesting that this repressor activity may be important for PIE-1 function in vivo.

In Caenorhabditis elegans, the predicted transcription factor SKN-1 is required for embryonic endodermal and mesodermal specification and for maintaining differentiated intestinal cells post-embryonically. The SKN-1 DNA-binding region is related to the Cap'n'Collar (CNC) family of basic leucine zipper proteins, but uniquely, SKN-1 binds DNA as a monomer. CNC proteins are absent in C. elegans, however; and their involvement in the endoderm and mesoderm suggests some functional parallels to SKN-1. Using a cell culture assay, we show that SKN-1 induces transcription and contains three potent activation domains. The functional core of one domain is a short motif, the DIDLID element, which is highly conserved in a subgroup of vertebrate CNC proteins. The DIDLID element is important for SKN-1-driven transcription, suggesting a likely significance in other CNC proteins. SKN-1 binds to and activates transcription through the p300/cAMP-responsive elementbinding protein-binding protein (CBP) coactivator, supporting the genetic prediction that SKN-1 recruits the C. elegans p300/CBP ortholog, CBP-1. The DIDLID element appears to act independently of p300/CBP, however, suggesting a distinct conserved target. The evolutionarily preservation of the DIDLID transcriptional element supports the model that SKN-1 and some CNC proteins interact with analogous cofactors and may have preserved some similar functions despite having divergent DNA-binding domains.During development, establishment of cell fates frequently involves conserved regulatory pathways. In the early Caenorhabditis elegans embryo, endodermal (intestinal) fates are specified by GATA family transcription factors (END-1, END-3, ELT-2) that are related to those that mediate endoderm development in Drosophila (Serpent) and vertebrates (GATA-4/5/6) (1-3). This program is triggered by maternally expressed SKN-1, a predicted transcription factor (4) that also specifies mesodermal lineages (pharynx and some body wall muscle) (5). The presence of consensus SKN-1-binding sites adjacent to the end-1 gene along with the timing of its expression suggests that SKN-1 may activate end-1 directly (2). SKN-1 is also required in the endoderm post-embryonically, to prevent differentiated intestinal cells from undergoing severe atrophy (5).Apparent SKN-1 orthologs have not been identified outside of nematodes, but in its DNA-binding region, SKN-1 is related to a subgroup of basic leucine zipper transcription factors (4, 5), the CNC 1 proteins. A basic DNA-binding region at the SKN-1 COOH terminus is particularly similar to those of CNC proteins, but SKN-1 lacks a zipper dimerization domain (Fig. 1, A and C) and, uniquely, binds DNA as a monomer (4). Its DNA binding requires the adjacent ␣-helical "CNC region" (Fig. 1, A and C) (4, 6 -8), which is otherwise found only in CNC proteins. An adjacent SKN-1 element that is lacking in CNC proteins, the NH 2 -terminal arm (Fig. 1, A and C), contributes additional binding affinity and specificity (6, 9). These similarities and differences pose t...

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