EXPRESSION OF SRY, THE MOUSE SEX-DETERMINING GENE

In the mouse, Sly is expressed by germ cells in the adult testis and b y somatic cells in the genital ridge. Transcripts in the former exist as circular RNA molecules of 1.23 kb, which are unlikely to be efficie ntly translated. We have used RNase protection to map the extent of th e less abundant Sry transcript in the developing gonad, We demonstrate that it is a linear mRNA derived from a single exon. This begins in t he unique region 5' of the protein coding region and extends several k ilobases into the 3' arm of the large inverted repeat which bounds the Sry genomic locus. Knowledge of this transcript, which is very differ ent from that of the human SRY gene, allows us to predict its protein product and reveals several features which may be involved in translat ional control. Our data is also consistent with there being two promot ers for the Sry gene, a proximal one that gives functional transcripts in the genital ridge and a distal promoter used in germ cells in the adult testis. As RNase protection is a quantitative technique, a detai led timecourse of Sly expression was carried out using accurately stag ed samples, Sry transcripts are first detectable just after 10.5 days post coitum, they reach a peak at 11.5 days and then decline sharply s o that none are detected 24 hours later. This was compared with anti-M ullerian hormone gene expression, an early marker of Sertoli cells and the first known downstream gene of Sly. Amh expression begins 20 hour s after the onset of Sry expression at a time when Sry transcripts are at their peak. While this result does not prove a direct interaction between the two genes, it defines the critical period during which Sry must act to initiate Sertoli cell differentiation.