ebg enzyme, the second beta-galactosidase of Escherichia coli, does not normally convert lactose into an inducer of the lac operon. We previously reported the existence of a mutant ebg enzyme that does make such an inducer in vivo (Rolseth et al., J. Bacteriol. 142:1036-1039, 1980). Here I report that the mutant enzyme makes inducer from lactose in vitro and that the inducer is allolactose. Allolactose is made from lactose by direct transgalactosylation at a rate that is 8 to 10% of the rate of lactose hydrolysis. Galactose is also transferred to glucose free in solution, but the resulting indirect transgalactosylation products are not allolactose or lactose. The ability to efficiently synthesize allolactose is a general property of class IV mutant ebg enzymes, whereas other classes of ebg mutant enzymes are unable to synthesize allolactose efficiently. The evolutionary implications of this new function are discussed
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