We have identified a gene, alternative testis transcripts (att), which is alternatively expressed, at both the RNA and protein levels, in testes and somatic tissues. The testis-specific RNA differs from somatic RNAs in both promoter usage and RNA processing and is dependent on the function of the transformer 2 gene. The differences between the somatic and testis RNAs have substantial consequences at the protein level. The somatic RNAs encode a protein with homology to the mammalian Graves' disease carrier proteins. The testis RNA lacks the initiation codons used in somatic tissue and encodes two different proteins. One of these begins in a testisspecific exon, uses a reading frame different from that for the somatic protein, and is completely novel. The other protein initiates translation in the frame of the somatic RNA at a Leu CUG codon which is within the open reading frame for the somatic protein. This produces a novel truncated version of the Graves' disease carrier protein-like protein that lacks all sequences N terminal to the first transmembrane domain.The genes of the sex determination regulatory cascade of Drosophila melanogaster affect all aspects of sex determination and sexual differentiation, including overt sexual differentiation, appropriate sexual behavior, and male and female fertility (see references 6,11,12,22,23,33,34,37, and 44 for some of many reviews). transformer 2 (tra2) is a key gene acting in the pathways controlling both somatic sexual differentiation and male fertility (7). In somatic tissues the tra2 protein, in conjunction with the female-specific transformer (tra) protein, regulates sex-specific splicing of doublesex (dsx) RNA (21,24,38,42,52,53). In addition to regulating dsx, tra2, again in conjunction with tra, also regulates at least one other gene that controls sexual behavior and sex-specific nervous system differentiation (47,48,50).tra2 is also necessary for male germ line function, although not for sex determination of the male germ line. Mutant males carrying loss-of-function tra2 mutations have morphologically normal germ lines but display incomplete spermatogenesis and produce immotile sperm (7). Two examples of tra2 regulation in the male germ line are known. Each exhibits both germ line-specific promoter usage and germ line-specific differences in RNA processing. One of these targets is tra2 itself. The tra2 protein regulates its own level by inhibiting the productive splicing of its own pre-mRNA (31). The exuperantia (exu) gene also shows a dependence on tra2 in the male germ line. In this case the requirement for tra2 is not absolute. Instead, the presence of tra2 ϩ seems to increase the efficiency of male germ line-specific processing events, including alternative splicing and polyadenylation (20).The product of the tra2 gene in D. melanogaster is a known RNA-binding protein (2,3,19,21,51,52). This binding depends on a single ribonucleoprotein motif in the central region of the protein. On dsx RNA, tra2 binds to a 300-bp region containing six copies of a 13-nucle...
