The replicative DNA helicase encoded by the dnaB gene is essential for chromosomal DNA replication in Escherichia coli. The DnaB protein is a component of the phi X-type primosome which is regarded as a model system for lagging strand synthesis of the chromosome. Using translational lacZ fusions at the plasmid and chromosomal levels, we studied the influence of DNA-damaging agents on dnaB gene expression. We found that DNA damage caused by mitomycin C, methyl methanesulphonate, 4-nitro-quinoline N-oxide, and UV irradiation led to a moderate, but significant induction of dnaB gene expression. Comparative S1 analysis of transcripts in untreated and induced cells demonstrated that the induction is due to increased transcription from the dnaB promoter. In contrast to other DNA damage-inducible replication genes, such as dnaA, dnaN, dnaQ, and polA, expression of which is not inducible in recA and lexA mutants, the induction of dnaB was also observed in a recA1 mutant. These results show that the induction of dnaB gene expression by replication-blocking DNA damage is due to a mechanism other than the indirectly SOS-dependent induction of the other DNA replication genes. Moreover, the data suggest that replication proteins are involved in recovery from replication-blocking DNA damage in two different ways--on the one hand at the level of initiation and on the other hand at the level of elongation.