Multiple pathways for SOS-induced mutagenesis in Escherichia coli: an overexpression of dinB/dinP results in strongly enhancing mutagenesis in the absence of any exogenous treatment to damage DNA

Abstract

dinP is an Escherichia coli gene recently identified at 5.5 min of the genetic map, whose product shows a similarity in amino acid sequence to the E. coli UmuC protein involved in DNA damage-induced mutagenesis. In this paper we show that the gene is identical to dinB, an SOS gene previously localized near the lac locus at 8 min, the function of which was shown to be required for mutagenesis of nonirradiated lambda phage infecting UV-preirradiated bacterial cells (termed lambdaUTM for lambda untargeted mutagenesis). A newly constructed dinP null mutant exhibited the same defect for lambdaUTM as observed previously with a dinB::Mu mutant, and the defect was complemented by plasmids carrying dinP as the only intact bacterial gene. Furthermore, merely increasing the dinP gene expression, without UV irradiation or any other DNA-damaging treatment, resulted in a strong enhancement of mutagenesis in F'lac plasmids; at most, 800-fold increase in the G6-to-G5 change. The enhanced mutagenesis did not depend on recA, uvrA, or umuDC. Thus, our results establish that E. coli has at least two distinct pathways for SOS-induced mutagenesis: one dependent on umuDC and the other on dinB/P.

Figure 1

Gene organization around the dinP/B gene. The coding regions are denoted by horizontal lines with a rightward arrowhead. The LexA binding sequence in the dinP/B promoter site and some of the restriction sites referred to in the text are indicated. The numbering of base positions follows that in the DNA database entry D38582. The HpaI (at 9239)-HindIII fragment obtained from the dinB1::MudI(Ap lac) mutant was subjected to DNA sequence analysis. The DNA regions (indicated at the right) carried by the plasmids are denoted by lines with arrowheads at both ends.

Figure 2

Identification of plasmid-coded proteins. The maxicell method of Sancar et al. (28) was used to label the proteins encoded by plasmids pYG751, pYG752, pYG753, pYG754, and pYG755 in the CSR603 strain with [³⁵S]methionine. Samples were subjected to electrophoresis on an SDS/14% polyacrylamide gel and visualized by fluorography.