THE MOUSE GENE ENCODING THE TESTIS-SPECIFIC ISOFORM OF POLY(A) BINDING-PROTEIN (PABP2) IS AN EXPRESSED RETROPOSON - INTIMATIONS THAT GENE-EXPRESSION IN SPERMATOGENIC CELLS FACILITATES THE CREATION OF NEW GENES

The gene encoding the testis-specific isoform of mouse poly(A) binding protein (Pabp2) has been isolated and sequenced. Unexpectedly, compar ison of the sequence of genomic and cDNAs demonstrated that the Pabp2 gene lacks introns, whereas all other functional Pabp genes in plants, amphibians, and mammals contain introns. Thus, the mouse Pabp2 gene i s a retroposon, created by synthesizing a reverse transcriptase copy o f a processed mRNA and inserting the copy into the genome. The Pabp2 r etroposon is unusual because it is functional: previous work demonstra tes that its promoter drives the accumulation of Pabp2 mRNA in meiotic and early haploid spermatogenic cells, and the Pabp2 mRNA encodes a p rotein whose size and RNA-binding specificities are characteristic of PABP in plants, yeast, and mammals (Kleene et al. 1994). Two novel fac tors can be implicated in the retention of function of the Pabp2 retro poson. First, the promoter of the Pabp2 gene is not derived from its i ntron-containing progenitor, Pabp1. Second, mRNAs encoding somatic PAB P isoform, PABP1, are present at high levels in meiotic and haploid sp ermatogenic cells. Both features contrast with the phosphoglycerate ki nase 2 retroposon, which is believed to compensate for the depletion o f the somatic isoform due to X-chromosome inactivation in meiotic sper matogenic cells. We also document that more functional retroposons are expressed in meiotic and haploid spermatogenic cells than in any othe r tissue and speculate that transcriptional derepression in spermatoge nic cells favors the creation of expressed retroposons.