umuC-Independent, recA-Dependent Mutagenesis

Abstract

Although targeted UV-radiation mutagenesis appears to require both the recA and umuC genes in Escherichia coli, examples of recA-dependent but umuC-independent mutagenesis exist, e.g., gamma-radiation mutagenesis (Mutat. Res. 128, 1, 1984) and streptozotocin mutagenesis (Mutat. Res. 166, 229, 1986). Most of the information on umuC-independent mutagenesis comes from studies on ionizing radiation mutagenesis. These results will be reviewed here. Analyses of the various suppressor and back mutations that result in argE3 and hisG4 ochre reversion and an analysis of trpE9777(+1 frameshift) reversion were performed on umuC and wild-type cells gamma irradiated in the presence and absence of oxygen. In wild-type cells, the presence of oxygen enhances gamma-radiation mutagenesis. Although the umuC strain showed the gamma-radiation induction of base substitution and frameshifts when irradiated in the absence of oxygen, the umuC mutation blocked all oxygen-dependent base-substitution mutagenesis, but not all oxygen-dependent frameshift mutagenesis. For anoxically-irradiated cells, the yields of GC→AT and AT→GC transitions were largely umuC independent, while the yields of (AT or GC)→TA transversions were heavily umuC dependent. Therefore, the data for anoxically-irradiated cells support the hypothesis that gamma irradiation produces two kinds of DNA lesions that require recA-dependent misrepair to induce mutations. For base-substitution mutagenesis, one kind of lesion requires the umuC gene and produces transversion mutations, while a second kind of lesion produces transition mutations and does not require the umuC gene. For cells irradiated in the presence of oxygen, there seems to be an additional kind of lesion whose mutagenic potential for base substitutions (but not frameshifts) is completely dependent on the umuC gene.