Recent evidence suggests that a putative nucleosome remodeling complex containing MEP-1, LET-418 and HDA-1 (the MEP-1 complex) mediates the inactivation of germline potential in somatic cells of the C. elegans embryo. 1 In the germline, the MEP-1 complex appears to be inhibited by PIE-1, a germline-specific zinc finger protein, which directly interacts with MEP-1. We propose that this repression of MEP-1 function is a part of the mechanism that maintains the unique chromatin organization essential for the germline development. In order to identify factors that mediate the MEP-1 repression, we are conducting a genetic screen based on the phenotype that results from ectopic expression of PIE-1 in somatic cells. Animals expressing PIE-1 from the
hsp16-41 (heat shock protein) promoter mimic the loss-of-function
mep-1 mutant, causing the derepression of PGL-1 expression in numerous somatic cells and penetrant synMuvB (synthetic multivulva) defect resulting from the deregulation of the vulval differentiation potential. We are carrying out this screen in conjunction with four secondary screens, which examine the levels of PIE-1 expression, potential defects in the germline development, the expression of PGL-1 in somatic cells, and the epistatic relationship with known synMuvB genes. Using this strategy, 73 mutants have been recovered from approximately 20,000 haploid genomes analyzed. We broadly categorize these mutants into two classes. Class I: 17 mutants strongly suppress the PIE-1-induced Muv phenotype but do not suppress the Muv phenotype induced by RNAi directed to
mep-1,
let-418,
lin-35(Rb),
lin-36,
lin-13 or
lin-53(RbAp48) . These are likely to be defective in the PIE-1 function itself (i.e. PIE-1 fails to inhibit the MEP-1/LET-418 complex) or in the negative regulation of the complex in somatic cells. Class II: 48 mutants suppress, partially or fully, the
mep-1(RNAi) -induced Muv phenotype. Some of these mutants are expected to show enhanced activities of other synMuvB components. While others derepress the genes normally repressed by the MEP-1 complex. Presumably, these genes are in turn important for the non-P6.p cells to adopt the vulval fate. Of the 48 Class II suppressors, 13 also suppress the Muv phenotype caused by
lin-35(Rb)(RNAi) , 12 suppress the
lin-36(RNAi)- induced Muv phenotype, and 4 suppress both the
lin-35(RNAi) - and
lin-36(RNAi) -induced Muv phenotype. 19 suppress neither
lin-35(RNAi) - nor
lin-36(RNAi) -induced Muv phenotype. These results suggest the presence of complex interactions among the synMuvB components, multiple critical targets regulated by the synMuvB pathway or both. We are primarily interested in the Class I mutants and some of the Class II mutants that specifically suppress the Muv phenotype induced by
mep-1(RNAi) . We will present the mapping, complementation, and genetic characterization of some of these suppressors. 1. Unhavaithaya et al., (2002) Cell , 111, 991-1002.