The B. subtilis translesion polymerase Pol Y1 is not strongly recruited to sites of replication upon different types of DNA damage

One challenge to DNA replication is the presence of unrepaired damage on the template strand, which can stall the replication machinery. This stall can be resolved by the translesion synthesis (TLS) pathway, in which specialized translesion polymerases are recruited to copy damaged DNA. Because TLS polymerases are error-prone, their activity is regulated at multiple levels to minimize unnecessary mutagenesis. Although the molecular mechanisms of bacterial TLS have been extensively studied in Escherichia coli, less is known about this pathway in other species. In E. coli, the TLS polymerase Pol IV is minimally enriched at replication forks in the absence of DNA damage but is strongly recruited upon replication stalling, enabling TLS while minimizing mutagenesis. However, we recently showed that the Bacillus subtilis TLS polymerase Pol Y1, the homolog of Pol IV, is moderately enriched near replication sites even during normal growth and is not further enriched upon treatment with the DNA damaging agent 4-nitroquinoline 1-oxide (4-NQO). It is unknown whether this behavior is unique to 4-NQO or general to other types of DNA damage. In this study, we investigate the effects of four different DNA damaging agents (ultraviolet light, methyl methanesulfonate, nitrofurazone, and mitomycin C) in B. subtilis. We first characterize the contributions of the two TLS polymerases, Pol Y1 and Pol Y2, to DNA damage survival and damage-induced mutagenesis after treatment with these agents. We then use single-molecule fluorescence microscopy to measure the localization and dynamics of individual Pol Y1 molecules in live B. subtilis cells. We find that Pol Y1 and Pol Y2 have differing effects on survival and mutagenesis, but that under no circumstances is Pol Y1 strongly recruited to sites of replication upon DNA damage. This study broadens our understanding of TLS in B. subtilis, indicating that there are notable differences in TLS mechanisms across bacteria.