Extensive genetic analysis has shown that
sdc-3 (formerly known as
dpy-29) controls hermaphrodite development by acting at the same posihon in the sex determination and dosage compensahon pathways as
sdc-1 and
sdc-2 (DeLong, Plenefisch, and Meyer).
sdc-3 is unique in that its sex determination and dosage compensation functions act independentiy and are separately mutable.
sdc-3 mutations that aflect only sex determinahon, only dosage compensation, or both processes have been obtained. In order to understand the nature of
sdc-3, as well as how it functions as a regulator of the sex determinahon and dosage compensation pathways, we have molecularly characterized this region. Both the sex determination and dosage compensation phenotypes exhibited by all ciasses of
sdc-3 alleles can be rescued by a single, 12 kbp genomic fragment encoding a mature transcript of 6778 nucleotides beginning with a SL-1 trans-spliced leader. The predicted translation product encodes a protein of 2150 amino acids spaced over a total of 15 exons. Developmental Northern analysis indicates that the
sdc-3 transcript is present only in eggs, L1 s, and L2s at approximately equal bvels. A total of 22 alleles of sdG3 have been characterized genebcally. Of these, we have determined the molecular nature of 19 alleles and found that there is a tight conrelahon between the phenotype caused by the allele and the actual position of the base change. Alleles that eliminate the dosage compensation function but not the sex determination funchon specifically eliminate a pair of zinc finger mohfs found at the C-terminus of
sdc-3. Mutations that eliminate the sex determination function but not the dosage compensation funchon result from missense mutations within a region having similarities to the ATP bnding domain of myosin. hnally, null alleles result from either deletions or termination codons that prevent translation prior to the sex determination domain. Site-directed mutations either within the myosin homology or the zinc finger motifs have been reintroduced into
sdc-3 mutant animals and assayed for the ability to control each pathway. An
sdc-3 protein that lacks 11 amino acids within the myosin homology fails to direct proper hermaphrodite development but properly regulates X-chromosome expression. In contrast, substitution of serines for cysteines within the zinc finger homology disnupts the sex determination function but not the dosage compensation function. In addihon, we have examined the effect of different classes of sdc- 3 mutations on the next gene in the sex determinahon pathway, the male- specific gene
her-1. Since
her-1 is known to be regulated at the level of mRNA accumulation (Trent, Schauer, and Wood), we have inveshgated whether mutahons in the sex determination funchon of
sdc-3 causes inappropriately high
her-1 mRNA levels in XX animals. By examining RNA made from either a weakly or a strongly masculinizing allele of
sdc-3, we have determined that these mutations do in fact cause overexpression of
her-1. DeLong and Meyer (see accompanying abstract) have shown that masculinizahon caused by mutahons in the sex determination portion of
sdc-3 are strongly suppressed by upsets in dosage compensation. Therefore, we have also investigated whether this suppression is evident at the level of
her-1 transcript accumulation by examining RNA made from strains mutant for both the sex determination function of
sdc-3 and a dosage compensation gene (dpy- 270r
dpy-28), or strains mutant for both the sex determinahon and dosage compensahon functions of
sdc-3. We have found that disruphons in dosage compensahon do in fact cause a decrease in the inappropriately high
her-1 mRNA levels caused by
sdc-3 sex determinahon mutations. We conclude that the sex determination function of
sdc-3 is required for the proper negative regulation of
her-1 mRNA levels in XX animals. However, defects in dosage compensation suppress the masculinization of these animals by restoring
her-1 mRNA levels to nearly normal.