c-mos proto-oncogene RNA transcripts in mouse tissues: structural features, developmental regulation, and localization in specific cell types.

c-mos RNA transcripts have been previously detected in mouse gonadal tissue and in late-term embryos. Here, we show that they are also present at low levels in placenta and in adult mouse brain, kidney, mammary gland, and epididymis. Marked differences are observed in the size of the mos RNA transcripts detected in different tissues. All transcripts appear to end at the same 3' position, and the tissue-specific size variations appear to be due to the use of different promoters. For example, the testicular and ovarian RNA transcripts initiate approximately 280 and approximately 70 base pairs, respectively, upstream from the first initiation codon, but both end at a common site downstream from the mos open reading frame. The expression of mos is developmentally regulated in gonadal tissue. Thus, the level of mos transcripts in testes is low for the first 3 weeks after birth, increases at least 10-fold around day 25, and reaches adult levels by day 30. In contrast, ovaries from preweaning mice contain a higher level of mos mRNA compared to ovaries from adult mice. In cell fractionation experiments we show that mos transcripts are present in haploid germ cells. We find that these transcripts are associated with monosomes and polysomes. The peculiar pattern of mos expression in mouse gonadal tissue suggests a role for the c-mos proto-oncogene in germ cell differentiation.