Two distinct forms of the 64,000 M_r protein of the cleavage stimulation factor are expressed in mouse male germ cells

Polyadenylation in male germ cells differs from that in somatic cells. Many germ cell mRNAs do not contain the canonical AAUAAA in their 3′ ends but are efficiently polyadenylated. To determine whether the 64,000 M_r protein of the cleavage stimulation factor (CstF-64) is altered in male germ cells, we examined its expression in mouse testis. In addition to the 64,000 M_r form, we found a related ≈70,000 M_r protein that is abundant in testis, at low levels in brain, and undetectable in all other tissues examined. Expression of the ≈70,000 M_r CstF-64 was limited to meiotic spermatocyles and postmeiotic spermatids in testis. In contrast, the 64,000 M_r form was absent from spermatocytes, suggesting that the testis-specific CstF-64 might control expression of meiosis-specific genes. To determine why the 64,000 M_r CstF-64 is not expressed in spermatocytes, we mapped its chromosomal location to the X chromosome in both mouse and human. CstF-64 may, therefore, be absent in spermatocytes because the X chromosome is inactivated during male meiosis. By extension, the testis-specific CstF-64 may be expressed from an autosomal homolog of the X chromosomal gene.