Glutamate and cyclic AMP regulate the expression of galactokinase in Mycobacterium smegmatis

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Abstract
It was found thatMycobacterium smegmatisis unable to utilize galactose as the sole carbon source because the sugar alone cannot induce galactokinase. However, galactokinase was induced by glutamate alone, and was further stimulated by galactose. Rifampicin completely inhibited the glutamate-mediated expression ofgalKin both the absence and presence of galactose. Extracellular cAMP stimulated the expression of the enzyme only in the presence of glutamate plus galactose. ThegalKgene fromM. smegmatis, including its upstream promoter region, was cloned in a plasmid inEscherichia coli. The expression of kinase from these clones inE. coliwas dependent on cAMP and its receptor protein (CRP). The expression of UDP-galactose 4-epimerase was constitutive. This and other evidence suggests that thegalKgene is not linked togalTandgalEin the mycobacterial genome. In a glutamate-independent galactose-utilizing mutant (gin-1mutant) ofM. smegmatis, galKwas expressed in the absence of both galactose and glutamate, while in the presence of galactose this expression was increased twofold in the absence of glutamate and fourfold in its presence. Extracellularly added cAMP reduced the expression of the enzyme in the presence of galactose plus glutamate nearly to the basal level. It is proposed that inM. smegmatisthegalKgene is expressed from two different promoters; the expression from one promoter is dependent on glutamate but not on galactose and cAMP, while that from the other requires all three components. The role of galactose is possibly to derepress the latter promoter.