Human testis-specific PGK gene lacks introns and possesses characteristics of a processed gene

Phosphoglycerate kinase (PGK) (ATP:3-phospho-D-glycerate 1-phosphotransferase, EC 2.7.2.3) is a metabolic enzyme functioning in the Embden-Meyerhof pathway that converts glucose (or fructose) to pyruvate. Two functional loci for the production of PGK have been identified in the mammalian genome. PGK-1 is an X-linked gene expressed constitutively in all somatic cells and premeitotic germ cells. The human PGK-1 gene consists of 11 exons and 10 introns encompassing a region approximately 23 kilobases (kb) in length. PGK-2 is an autosomal gene expressed in a tissue-specific manner exclusively in the late stages of spermatogenesis. In the present study, a molecular analysis of a human genomic clone of PGK-2 originally isolated by Szabo et al. has revealed that this autosomal sequence completely lacks introns and contains characteristics of a processed gene, or 'retroposon', including the remnants of a poly(A)+ tail and bounding direct repeats. Typically such processed sequences form non-functional pseudogenes that have evolved multiple genetic lesions which preclude translation of any transcript into a functional polypeptide. For example, an X-linked processed pseudogene of PGK-1 (psi PGK-1) in humans has been identified and shown to contain premature termination codons in all reading frames. It was therefore unexpected to find that the intronless autosomal PGK sequence reported here is not a pseudogene, but is rather a functional gene that has retained a complete open reading frame, and is actively expressed in mammalian spermatogenesis. Both the unusual conservation of function in this processed PGK-2 gene and its tissue-specific expression in spermatogenesis are best explained as a compensatory response to the inactivation of the X-linked PGK-1 gene in spermatogenic cells before meiosis.