The control of -galactosidase specified by the lactose transposon Tn951 (inserted into RP1 to give pGC9114) has been studied in K12, ; in the first two species comparison could be made with F. In K12, the Tn951 and chromosomally encoded enzymes showed marked qualitative differences in regulation, the former giving a substantially lower maximum induced level and induction ratio. Several parameters were slightly affected by strain background. In , -galactosidase control determined by both F (in accord with earlier work) and pGC9114 was markedly different from in that maximal induced levels were about an order of magnitude lower and the induction ratio was reduced to 3 to 5. In and , Tn951-specified expression was qualitatively similar to that in . Possible reasons for anomalous expression in and are discussed.


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