Structural analysis of a hmg-coA-reductase pseudogene: insights into evolutionary processes affecting the hmgr gene family in allotetraploid cotton (Gossypium hirsutum L.).

Structural analysis of hmg-coA reductase (hmgr) genes in the allotetraploid cotton species Gossypium hirsutum L. revealed the first-known existence of a pseudogene, psihmg5, for this important enzyme. Complete sequencing of the genomic clone hmg5 unveiled several deleterious lesions, resulting in an organization that departed significantly from the linear canonical hmgr gene structure. Although analysis of the 5' flanking region indicated a promoter-like composition based on comparison with other known plant hmgr genes, the precise loss of intron 3, and putative poly-(A) signals, small poly-(A) tracts, and terminal repeats (TRs) found in the 3'-flanking region are characteristic features of retro-pseudogenes. DNA-blot analysis indicated that a psihmg5-related subfamily exists within a larger hmgr gene family in cotton. Several mechanisms are proposed to account for the formation of this partially intronless pseudogene, including intragenic homologous-replacement recombination and gene conversion involving a cDNA. Alignments of psihmg5 with functional cotton homologs also raised interesting possibilities for the formation of 'chimeric' gene structures, or differential intragenic mutation rates, as potential evolutionary mechanisms involved in shaping the hmgr gene family in cotton.