EXPRESSION AND PROPERTIES OF WILD-TYPE AND MUTANT FORMS OF THE DROSOPHILA SEX COMB ON MIDLEG (SCM) REPRESSOR PROTEIN

The Sex comb on midleg (Scm) gene encodes a transcriptional repressor of the Polycomb group (PcG). Here we show that SCM protein is nuclear and that its expression is widespread during fly development. SCM prot ein contains a C-terminal domain, termed the SPM domain, which mediate s protein-protein interactions. The biochemical function of another do main consisting of two 100-amino-acid-long repeats, termed ''mbt'' rep eats, is unknown. We have determined the molecular lesions of nine Scm mutant alleles, which identify functional requirements for specific d omains. The Son alleles were tested for genetic interactions with muta tions in other PcG genes. Intriguingly, three hypomorphic Son mutation s, which map within an mbt repeat, interact with PcG mutations more st rongly than do Scm null alleles. The strongest interactions produce pa rtial synthetic lethality that affects doubly heterozygous females mor e severely than males. We show that mbt repeat alleles produce stable SCM proteins that associate with normal sites in polytene chromosomes. We also analyzed progeny from Scm mutant germline clones to compare t he effects of an mbt repeat mutation during embryonic vs. pupal develo pment. We suggest that the mbt repeat alleles produce altered SCM prot eins that incorporate into and impair function of PcG protein complexe s.