Methyl galactosidase activity: an alternative evolutionary destination for the ebgA0 gene.

Previous studies (Campbell et al., 1973; Hall and Hartl, 1974; Hall and Hartl, 1975) have shown that the ebgA0 gene, whose product does not hydrolyze lactose may evolve so that its product does hydrolyze lactose; i.e., lactase activity is one evolutionary destination of the ebgA0 gene. Beginning with a strain that synthesizes ebgA0 gene product constitutively and grows extremely slowly (doubling time, 30 to 50 h) on methyl-beta-D-galactopyranoside (MG), a derivative was selected capable of growth on MG at a moderate rate (doubling time, 5.9 h). Genetic evidence is presented showing that the gene that permits growth on MG is an allele of ebgA. A comparison among strains bearing several alleles of ebgA shows that the new allele, termed ebgAmg, synthesizes a product specific for MG and thus represents a true alternative evolutionary destination for the ebgA0 gene.