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^1-4.. The human PGK-1 gene consists of 11 exons and 10 introns encompassing a region ∼23 kilobases (kb) in length ⁵. PGK-2 is an autosomal gene expressed in a tissue-specific manner exclusively in the late stages of spermatogenesis^3,4,6-8. In the present study, a molecular analysis of a human genomic clone of PGK-2 originally isolated by Szabo et al.⁹ has revealed that this autosomal sequence completely lacks introns and contains characteristics of a processed gene ^10,11, or 'retroposon'^12,13, 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 polypeptide¹⁰. For example, an X-linked processed pseudogene of PGK-1 (ψPGK-1) in humans has been identified and shown to contain premature termination codons in all reading frames¹⁴. 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.