Indirect and intragenic suppression of the lexA102 mutation in E. coli B/r

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 ability to express SOS functions. These mutants were isolated as UV resistant UV nonmutable (Rnm) derivatives of the lexA102 uvrA155 mutant strain WP51. The UV resistance of these Rnm strains is a result of the suppression of lexA102 mediated UV sensitivity. 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 constitutive 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 expression 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.