RESPONSE TO UV DAMAGE BY 4 ESCHERICHIA-COLI K-12 RESTRICTION SYSTEMS

To understand the role of restriction in regulating gene how in bacter ial populations, we would like to understand the regulation of restric tion enzyme activity. Several antirestriction (restriction alleviation ) systems are known that reduce the activity of type I restriction enz ymes like EcoKI in vivo. Most of these do not act on type II or type I II enzymes, but little information is available for the unclassified m odification-dependent systems, of which there are three in E. coli K-1 2. Of particular interest are two physiological controls on type I enz ymes: EcoKI restriction is reduced 2 to 3 orders of magnitude followin g DNA damage, and a similar effect is seen constitutively in Dam(-) ce lls. We used the behavior of EcoKI as a control for testing the respon se to W treatment of the three endogenous modification-dependent restr iction systems of K-12, McrA, McrBC, and Mrr. Two of these were also t ested for response to Dam status. We find that all four resident restr iction systems show reduced activity following UV treatment, but not i n a unified fashion; each response was genetically and physiologically distinct. Possible mechanisms are discussed.