Evidence for the inducibility of the uvrB operon

A new tool for probing the regulation of non-essential gene products has recently been developed 1 —this is a derivative of bacteriophage Mu into which the Escherichia coli genes for lactose metabolism have been fused (referred to here as Mu(lac, amp)). The DNA of this defective virus can insert randomly into the chromosome of a cell and mutants canbe retrieved in which the β -galactosidase gene carried on the phage is under the control of the regulatory elements of the operon in which it now resides.We used this phage to generate DNA repair-deficient mutants, one of which washighly UV-sensitive due to insertion of the Mu(lac, amp) into the uvrB regionof the chromosome. When exposed to UV light, this strain produces β -galactosidase at a much greater rate than unexposed cells. This suggests that the uvrB operon is inducible. These results were initially rather surprising. Although various components of the enzyme systems involved in Weigle-reactivation 2,3 , post-replication repair 4–6 and repair of alkylation damage 7,8 had been established as inducible, the products of the uvrA , uvrB and uvrC genes which collectively effect nucleotide excision repair 9–11 , seemed to be firmly established as a constitutively synthesized repair system. This conclusion was drawn because the activity of these enzymes can be easily detected in uninduced cells 9–11 ; and mutations in the genes recA and lexA , which affect the regulation of most of the inducible repair systems 12 , have relatively little effect on uvrA , uvrB -mediated excision repair 13 . Thus the uvrA and uvrB gene products seemed to be produced constitutively at maximal or near maximal rates. However, recent reports suggest a role for the uvrA and uvrB gene products in various inducible repair systems 6,14–17 . Because these studies all relied on indirect measurements of induction, it could never be determined if the results reflected induction of other components of these systems or if the uvrA or uvrB gene products themselves were regulated. The insertion mutant described here allowed us to monitor expression of the uvrB operon directly and provided evidence for regulated synthesis of the uvrB gene product.