STRUCTURAL-ANALYSIS OF A HMG-COA REDUCTASE PSEUDOGENE - INSIGHTS INTOEVOLUTIONARY PROCESSES AFFECTING THE HMGR GENE FAMILY IN ALLOTETRAPLOID COTTON (GOSSYPIUM-HIRSUTUM L.)

Structural analysis of hmg-coA reductase (hmgr) genes in the allotetra ploid cotton species Gossypium hirsutum L. revealed the first-known ex istence of a pseudogene, psi hmg5, for this important enzyme. Complete sequencing of the genomic clone hmg5 unveiled several deleterious les ions, resulting in an organization that departed significantly from th e linear canonical hmgr gene structure. Although analysis of the 5' fl anking region indicated a promoter-like composition based on compariso n with other known plant hmgr genes, the precise loss of intron 3, and putative poly-(A) signals, small poly-(A) tracts, and terminal repeat s (TRs) found in the 3'-flanking region are characteristic features of retro-pseudogenes. DNA-blot analysis indicated that a psi hmg5-relate d subfamily exists within a larger hmgr gene family in cotton. Several mechanisms are proposed to account for the formation of this partiall y intronless pseudogene, including intragenic homologous-replacement r ecombination and gene conversion involving a cDNA. Alignments of psi h mg5 with functional cotton homologs also raised interesting possibilit ies for the formation of 'chimeric' gene structures, or differential i ntragenic mutation rates, as potential evolutionary mechanisms involve d in shaping the hmgr gene family in cotton.