We describe the properties of a new gene,
sop-3, that is required for the regulated expression of a C. elegans Hox gene,
egl-5, in a postembryonic neuroectodermal cell lineage. Regulated expression of
egl-5 in this cell lineage is necessary for development of the sensory rays of the male tail,
sop-3 encodes a predicted novel protein of 1475 amino acids without clear homologs in other organisms. However, the sequence contains motifs consisting of homopolymeric runs of amino acids found in several other transcriptional regulators, some of which also act in Hox gene regulatory pathways. The genetic properties of
sop-3 are very similar to those of
sop-1, which encodes a component of the transcriptional Mediator complex, and mutations in the two genes are synthetic lethal. This suggests that SOP-3 may act at the level of the Mediator complex in regulating transcription initiation, In a
sop-3 loss-of-function background,
egl-5 is expressed ectopically in lineage branches that normally do not express this gene. Such expression is dependent on the Hox gene
mab-5, as it is in branches where
egl-5 is normally expressed, Ectopic
egl-5 expression is also dependent on the Wnt pathway. Thus,
sop-3 contributes to the combinatorial control of
egl-5 by blocking
egl-5 activation by MAB-5 and the Wnt pathway in inappropriate lineage branches.