Lactose metabolism in Erwinia chrysanthemi

Abstract

Wild-type strains of the phytopathogenic enterobacterium Erwinia chrysanthemi are unable to use lactose as a carbon source for growth although they possess a β-galactosidase activity. Lactose-fermenting derivatives from some wild types, however, can be obtained spontaneously at a frequency of about 5 x 10-7. All Lac+ derivatives isolated had acquired a constitutive lactose transport system and most contained an inducible β-galactosidase. The transport system, product of the lmrT gene, mediates uptake of lactose in the Lac+ derivatives and also appears to be able to mediate uptake of melibiose, raffinose, and galactose. Two genes encoding β-galactosidase enzymes were detected in E. chrysanthemi strains. That mainly expressed in the wild-type strains was the lacZ product. The other, the lacB product, is very weakly expressed in these strains. These enzymes showed different affinities for the substrates o-nitrophenyl-β-galactopyranoside and lactose and for the inhibitors isopropyl-β-D-thiogalactopyranoside and galactose. The lmrT and lacZ genes of E. chrysanthemi, together with the lacI gene coding for the regulatory protein controlling lacZ expression, were cloned by using an RP4::miniMu vector. When these plasmids were transferred into Lac- Escherichia coli strains, their expression was similar to that in E. chrysanthemi. The cloning of the lmrT gene alone suggested that the lacZ or lacB gene is not linked to the lmrT gene on the E. chrysanthemi chromosome. One Lac+ E. chrysanthemi derivative showed a constitutive synthesis of the β-galactosidase encoded by the lacB gene. This mutation was dominant toward the lacI lacZ cloned genes. Besides these mutations affecting the regulation of the lmrT or lacB gene, the isolation of structural mutants unable to grow on lactose was achieved by mutagenic treatment. These mutants showed no expression of the lactose transport system, the lmrT mutants, or the mainly expressed β-galactosidase, lacZ mutants. The lacZ mutants retained a very low β-galactosidase level, due to the lacB product, but this level was low enough to permit use of the lacZ mutants for the construction of gene fusions with the Escherichia coli lac genes.