Contrasting patterns of nucleotide polymorphism suggest different selective regimes within different parts of the gene in L.

BACKGROUND

Phosphoglucose isomerase (PGI, EC 5.3.1.9) is an essential metabolic enzyme in all eukaryotes. An earlier study of the gene, which encodes cytosolic PGI in the grass L., revealed a marked difference in the levels of nucleotide polymorphism between the 5' and 3' portions of the gene.

METHODS

In the present study, we characterized the sequence polymorphism in in more detail and examined possible explanations for the non-uniform pattern of nucleotide polymorphism across the gene.

RESULTS

Our study confirms that the two portions of the gene show substantially different levels of DNA polymorphism and also suggests that the peptide encoded by the 3' portion of is functionally and structurally more important than that encoded by the 5' portion. Although there was some evidence of purifying selection (/ test) on the 5' portion of the gene, the signature of purifying selection was considerably stronger on the 3' portion of the gene (/ and McDonald-Kreitman tests). Several tests support the action of balancing selection within the 5' portion of the gene. Wall's and tests were significant only for the 5' portion of the gene. There were also marked peaks of nucleotide diversity, Tajima's and the / ratio at or around a codon site (within the 5' portion of the gene) that a previous study had suggested was subject to positive diversifying selection.

CONCLUSIONS

Our results suggest that the two portions of the gene have been subject to different selective regimes. Purifying selection appears to have been the main force contributing to the relatively low level of polymorphism within the 3' portion of the sequence. In contrast, it is possible that balancing selection has contributed to the maintenance of the polymorphism within the 5' portion of the gene.