Analysis of Melibiose Mutants Deficient in α-Galactosidase and Thiomethylgalactoside Permease II in Escherichia coliK-12

Three types of mutants (mel−) unable to metabolize the α-d-galactoside, melibiose, were derived from Escherichia coliK-12. One type lacked α-galactosidase; another lacked a specific transport system, termed thiomethylgalactoside (TMG) permease II; and the third lacked both of these functions. The mutational sites were genetically mapped by recombination frequency with different markers and by determination of chromosomal transfer in interrupted-mating experiments. All three mel−mutant types mapped in a cluster near to the metAmarker on the E. colichromosome and were cotransducible. Induction studies revealed that the three α-d-galactosides, melibiose, melibiitol, and galactinol, induced α-galactosidase and TMG permease II coordinately; d-galactose also induced them but only in a galactokinaseless mutant. These data suggest that α-galactosidase and TMG permease II may be components of a common operon.