Characterization of a constitutive variant of the murine serum protein allotype, Slp.
Properties of the Ss serum protein (putatively the murine homologue of the fourth component of complement) and its allotypic form, Slp, were studied in a wild-derived variant, Ssomega7. The Ssomega7 allele is unique in that it confers non-sex-limited expression of the Slp allotype. During examination of the H-2 haplotypes of two congenic lines which derived their Ssomega7 allele from a common origin, it was found that intra-H-2 recombination had occurred during the establishment of one of these lines. The sex-limited expression of Slp as determined by the Ssd allele was constrasted in several ways to the constitutive expression of Slp as determined by the Ssomega7 allele. Slp expression in mice carrying the Ssd allele was found to be subject to control by the testicular feminization mutant, Tfm, while in mice carrying Ssomega7 the Slp expression was independent of Tfm control. This implies that Slp is subject to a rather fundamental hormonal control mechanism in mice carrying Ssd and presumably all other Slp-positive Ss alleles, with the exception of Ssomega7. It is proposed that Ssomega7 may represent an "operator-constitutive" mutation of the Jacob-Monod type. Including the Ssomega7 allele, there are now at least seven distinguishable Ss variants. Each of these Ss alleles is characterized by a unique level of Ss and/or Slp antigenic expression. The sex-limitation difference and quantitative differences in Ss and Slp expression associated with the Ssomega7 allele were found to be codominant or intermedaite when Ssomega7 was in heterozygous combinations with other Ss alleles. This indicates a lack of trans effects and argues against genetic differences in regulatory proteins as the basis for these polymorphisms.