Martin Victor et al. (2001) International C. elegans Meeting "A POP-1 repressor complex restricts inappropriate cell type-specific gene transcription during C. elegans embryogenesis"

Dominica Calvo, Morris Maduro, Margaret Po-Shan Luke, Yang Shi, Guangchao Sui, Joel Rothman, Martin Victor, Gengyun Wen

[International C. elegans Meeting, 2001]

During animal development, cell fate determination is a critical decision process that sets the stage for future differentiation of specific cell types. In C. elegans , previous studies have shown that histone acetyltransferase CBP-1 counteracts the repressive activity of the histone deacetylase HDA-1 to allow endoderm differentiation, which is specified by the E cell. In the sister MS cell, the endoderm fate is prevented by the action of an HMG box-containing protein POP-1 through an unknown mechanism. In this study, we show that the actions of CBP-1, HDA-1 and POP-1 converge on end-1 , an initial endoderm-determining gene. In the E lineage, CBP-1 is required to overcome inhibition by both HDA-1 and POP-1 in order to activate end-1 . We further identify a molecular mechanism for the endoderm-suppressive effect of POP-1 by demonstrating that POP-1 functions as a transcriptional repressor that inhibits inappropriate end-1 transcription. We provide biochemical and functional evidence that POP-1 represses transcription via the recruitment of HDA-1 and UNC-37, the C. elegans homolog of the co-repressor Groucho. These findings demonstrate the importance of the interplay between acetyltransferases and deacetylases in the regulation of a critical cell fate-determining gene during development. Furthermore, these results identify a strategy by which concerted actions of histone deacetylases and other co-repressors ensure maximal repression of inappropriate cell type-specific gene transcription. This combinatorial scheme appears to be conserved in multiple organisms.

Zhang, Yinhua et al. (2009) International Worm Meeting "Genetic Study of Translation Start Codon Recognition in C. elegans."

Maduzia, Lisa, Poullet, Nausicaa, Charffre, Sebastien, Zhang, Yinhua

[International Worm Meeting, 2009]

Recognition of AUG start codon during translation initiation is critical for decoding the genetic information on mRNAs. In prokaryotes, the Shine-Dalgarno sequence located 5'' to the AUG on the mRNA complements to a region on the 16S rRNA and this base-pairing interaction positions the mRNA so that the initiator tRNA finds the AUG efficiently. In contrast, there is no evidence in eukaryotes that such base-paring occurs, and instead eukaryotes appear to rely mainly on the base pairing of the AUG with the anticodon of the initiator tRNA. It is postulated that recognition is achieved through ribosomal scanning, in which a pre-initiation complex, consisting of the small ribosomal subunit, multiple translation initiation factors (eIFs) and initiator tRNA, moves along the mRNA to examine each codon by base pairing with the anticodon. Perfect base pairing triggers the hydrolysis of GTP by the heterotrimeric G-protein eIF2 and leads to a cascade of molecular interactions that allow translation to start. We have developed a genetic system in C. elegans to study how the start codon AUG is recognized. Our strategy is similar to that employed in the isolation of S. cerevisiae sui (suppressor of initiation codon) mutants, whereby a selectable reporter that contained an altered initiation codon was utilized (Donahue et al., Cell 54:621, 1988). We changed the ATG translation initiation codon of an abundantly expressed GFP reporter to that of a GTG codon and found that the GFP signal is almost completely eliminated in wild type worms. With this silent reporter, we performed genetic screens and obtained mutants that showed varying degrees of GFP expression. Mutations in two mutant strains have been identified. Both are missense mutations; one in the b subunit of eIF2 and the other in eIF1. We further studied if this newly identified eIF2b mutation and the yeast SUI3 mutations when introduced into the corresponding sites of the C. elegans gene could initiation translation with other non-AUG codons. We found that all of these mutants allow GFP to be synthesized from reporters that contain altered start site codons that differ by one base either in the first or third base position. Changes in the second base position result in much less GFP expression and changes of two or three bases reduce expression to undetectable levels. Our results differ from what was previously observed in S. cerevisiae. The SUI3 mutations only allow translation from the non-AUG codon UUG in the yeast reporter assay (Huang et al., Genes Dev 11:2396, 1997). Our analyses in C. elegans suggest that eIF2b plays a role in recognizing proper codon-anticodon pairing and/or in relaying the signal of correct base pairing which leads to GTP hydrolysis.