Molec. gen. Genet. 177, 129 137 (1979) © by Springer-Verlag1979 Indirect and Intragenie Suppression of the lexAl02 Mutation in E. coli B/r Michael R. Volkert 1, David F. Spencer 2., and Alvin J. Clark 1 1 Department of Molecular Biology,Universityof California, Berkeley,California94720, USA 2 Department of Biochemistryand Biophysics,Pennsylvania State University,UniversityPark, Pennsylvania16802, USA Summary. In Escherichia coli B/r the expression of UV inducible (SOS) functions is under the control of the recA and lexA genes. In this study we have characterized mutants which are altered in their abil- ity to express SOS functions. These mutants were isolated as UV resistant UV nonmutable (Rnm) deri- vatives of the lexAl02 uvrA155 mutant strain WP51. The UV resistance of these Rnm strains is a result of the suppression of lexAl02 mediated UV sensitiv- ity. Genetic mapping of rnm mutations shows that the two predominant classes, rnmA and rnmB, map in or very near the lexA and recA genes respectively. rnmA mutations differ from rnmB with regard to recA protein synthesis, rnmA mutations do not restore the ability to express high levels of recA protein after UV treatment whereas rnmB mutations result in con- stitutive expression of high levels of recA protein. However, both rnmA and rnmB mutant strains inhibit postirradiation DNA degradation. This shows that in rnmA strains, high levels of recA protein are not needed to inhibit postirradiation DNA degradation. The genetic map location and constitutive expres- sion of recA protein synthesis resulting from rnmB mutations suggests that they are operator constitutive mutations of the recA gene. The result that the lexA + gene is required for the expression of UV mutagenesis in rnmB mutants shows that high levels of recA protein do not circumvent the need for the lexA + gene product in this process. Thus, while the lexA gene product is required for the induction of recA protein synthesis, lexA must have an additional role in UV induced mutagenesis. * Present address." Department of Biochemistry, Dahlhousie University, Halifax, Nova Scotia, Canada B3H 4H7 Send offprint requests to : M.R. Volkert Introduction Treatment of Escherichia coli with ultraviolet (UV) light or other agents which damage DNA or interrupt its synthesis results in the expression of a group of diverse functions now known collectively as SOS functions (Radman, 1974; Witkin, 1976). All of these functions are under the control of the recA and lexA genes, and are believed to promote the survival of irradiated cells or phage (Defais et al., 1971 ; Radman, 1974; Witkin, 1974; Witkin, 1976; Volkert etal., 1976). Included among the list of SOS functions are : filamentous growth, lambda prophage induction, er- ror-prone DNA repair (SOS repair), W-reactivation and mutagenesis of irradiated phage, inhibition of DNA degradation, induction of the recA protein (for- merly called protein X), inhibition of respiration (for references see Witkin, 1976), and alleviation of restric- tion (Day, 1977). The recA protein appears to have at least two and possibly more activities. One activity catalyzes the homologous pairing of single stranded DNA (Weinstock etal., 1979; Shibata, etal., 1979), and the pairing of single stranded DNA with homologous superhelical DNA (Shibata et al., 1979). This activity appears to be involved in synapsis of parental DNA elements, a step required for genetic recombination. The second activity of the recA protein which has been identified is a protease activity. This activity causes the proteolytic cleavage and inactivation of the repressor of lambda prophage (Roberts and Ro- berts, 1975; Roberts et al., 1977). In addition to caus- ing the induction of lambda prophage, it has been proposed that this activity of the recA protein may cause the induction of all other SOS functions. A third activity has been proposed for the recA protein (Gudas and Pardee, 1975; Satta etal., 1979). This proposal is based on evidence which suggests that the recA protein may act as the inhibitor of postirradi- 0026-8925/79/0177/0129/$01.80