Enzyme activity in ems-induced null mutations of duplicated genes encoding phosphoglucose isomerases in clarkia

The duplication of the nuclear gene encoding the cytosolic isozyme of phosphoglucose isomerase (PGI; EC 5.3.1.9) originated within Clarkia, a genus of annual plants native to California. Previous immunological studies showed that species with and without the duplication have the same levels of cytosolic PGI activity (relative to that of the plastid PGI isozyme), as well as similar levels of cytosolic PGI protein. In the present study, we characterized seven EMS-induced null activity mutations in both duplicate PGI genes. The mutations reduced PGI activity levels in direct proportion to the normal contribution of each gene. Homozygous mutants at Pgi-3 had 64% of wild-type activity, whereas those at Pgi-2 had only 36%. The effects of the mutations at the two loci were additive, as shown by further reductions in activity in certain progeny classes in F(2) progenies between them. The homozygous double null mutant class was not recovered and is presumably lethal. All of the mutants appear to be CRM+. The results account for the previously observed differences in in vivo accumulation of the duplicate isozymes in numerous Clarkia species. The results further show that PGI activity is not directly regulated by metabolic factors and suggest that the reduced PGI levels in Clarkias with the duplication probably evolved by regulatory changes in transcription or translation. The study also demonstrates a novel method to evaluate whether a particular enzyme activity is essential.