The Phosphoglycerate Kinase (Pgk) Gene Family – An Example of Evolution by Gene Duplication

The tissue-specific phosphoglycerate kinase-2 (Pgk2) gene encodes a glycolytic enzyme necessary for sperm function and male fertility.  The autosomal Pgk2 gene derived from the X-linked Pgk1 gene via retroposition, and is expressed uniquely in meitotic and postmeiotic spermatogenic cells.  We hypothesize that the Pgk2 retrogene diverged from the ubiquitously expressed Pgk1 gene to adopt testis-specific expression and specialized, sperm-specific functions.  To test this hypothesis, we examined Pgk gene sequences from eutherian mammals (e.g. human, mouse and others), to assess conservation of promoter regions regulating ubiquitous or tissue-specific transcription, and elements in the 3’-UTR region regulating post-transcriptional stabilization of the RNA product.  Among eutherians, we found that Pgk1 promoters have conserved a CpG-island, a pair of GC-boxes, a pair of CAAT-boxes, and an NF-1 sequence, while Pgk2 promoters have uniformly lost the CpG-island, retained a single GC-box and a single CAAT-box, and gained an enhancer sequence (E3/E4) with homology to an NF-1 element.  We also found that Pgk2 3’-UTRs have two PTBP2 elements not present in the corresponding subregion of the Pgk1 3’-UTRs.  Our results support our hypothesis that the Pgk2 gene evolved unique sequence elements that regulate tissue-specific transcription and post-transcriptional transcript stabilization, and that these features have contributed to functional specializations that distinguish the Pgk2 retrogene from the Pgk1 gene as predicted by Ohno’s hypothesis of evolution by gene duplication.  We are currently examining Pgk gene sequences from metatherian (opossum and tammar) and prototherian (platypus) species to extend our understanding of the evolutionary history of the mammalian Pgk genes.