Functional effects of PGI allozymes in Escherichia coli

Five alleles representing three electromorphs of phosphoglucose isomerase (PGI) have been transferred from natural isolates of E. coli into the genetic background of E. coli K12 and examined for their effect on growth rate in chemostats limited for glucose or fructose. With glucose limitation, all alleles are selectively neutral or nearly neutral within the limit of resolution of the technique, whether the genetic background is nonmutant or whether it contains a deletion of the locus of glucose-6-phosphate dehydrogenase, the enzyme that provides an alternative metabolic pathway for the substrate of PGI. With fructose limitation, one of the naturally occurring alleles has a small but reproducible detrimental effect on growth rate. A kinetic difference in this detrimental allozyme, apparently relating to an inhibition constant, has been observed in some, but not all, lots of substrate, and a similar difference has also been noted in one of the rare electromorphs that could not be transferred into E. coli K12. These results support a model of genetic variation in which the alleles are neutral or nearly neutral in the prevailing environment but have a potential for selection that can be expressed under the appropriate conditions of environment or genetic background. This hypothesis is discussed in the context of allozyme polymorphisms observed in other organisms.