The synthetic Multivulva (synMuv) genes are grouped into two functionally redundant classes, A and B, that negatively regulate the induction of vulval cell fates. Animals with a mutation in one or more genes within the same class are non-Muv. By contrast, animals with mutations in both a class A and a class B gene are Muv. Among some of the identified class B gene products are counterparts of a transcriptional repression complex: Rb (LIN-35), the Rb-associated protein RbAp48 (LIN-53), the heterodimeric transcription factor DP/E2F (DPL-1/EFL-1), and the chromatin-regulating proteins HDAC (HDA-1), Mi-2 (LET-418), and HP1 (HPL-2).To understand better the synMuv A and B pathways, two screens for synMuv suppressors were performed. In these screens, 166 suppressors of the synMuv phenotype of lin-15AB
(n765ts) and 43 suppressors of the
lin-53(
n833);
lin-15A(
n767) synMuv phenotype were isolated. One
lin-53(
n833);
lin-15A(
n767) synMuv suppressor was cloned and found to encode a homolog of the chromatin-remodeling ATPase ISWI. RNAi or a presumptive null allele of this gene, which we call
isw-1, suppresses the Muv phenotype of most, if not all, synMuv mutant combinations. The loss-of-function Vul phenotype of mutants in the RTK/Ras pathway (except
lin-3 EGF) is epistatic to the synMuv phenotype. Loss of
isw-1 function cannot suppress the Muv phenotype of
lin-1 mutants or a gain-of-function mutation in
let-60. We conclude that
isw-1 is acting downstream of or in parallel to one or both synMuv classes and upstream of or in parallel to the RTK/Ras pathway.ISWI has been biochemically purified from Drosophila embryo extracts as a member of multiple complexes, including the Nucleosome Remodeling Factor (NURF), the Chromatin Accessibility Complex (CHRAC), and the ATP-utilizing Chromatin assembly and remodeling Factor (ACF). RNAi of C. elegans homologs of NURF, CHRAC, and ACF complex members led us to identify a NURF301 homolog, F26H11.2, as a gene that likely acts with
isw-1 to regulate vulval development. NURF301 is the largest subunit of the NURF complex and is implicated in complex formation, stability and interactions with transcription factors. We are seeking a deletion allele of F26H11.2 to further characterize the activity of the putative NURF complex in antagonism of the synMuv phenotype. Furthermore, we are mapping and cloning additional mutationally-defined suppressors of the synMuv phenotype in an effort to identify other factors that may act with ISW-1 or a putative C. elegans NURF complex.