Influence of uvrA, recJ and recN gene mutations on nucleoid reorganization in UV-treated Escherichia coli cells

Exposure to ultraviolet (UV) radiation blocks DNA replication and arrests cellular division in Escherichia coli. Restoration of chromosome replication involves nucleoid reorganization, which involves the participation of the recombination-catalyzing proteins RecA, RecO, RecR and RecN. In this work, we evaluated the influence of recN, uurA and red gene mutations on post-irradiation nucleoid reorganization. We used isogenic E. coli strains that are defective for these genes to study post-irradiation kinetics of the nucleoid shape fractions using fluorescence microscopy. The results showed that in the wild-type strain, post-irradiation nucleoid reorganization occurs, which restores the nucleoid shape fractions in the cells to those observed prior to irradiation. First, the nucleoid condenses into the central area of the irradiated cell. Second, the nucleoid disperses along the cell. Third, the cell enters the chromosome replicative phase and cytokinesis. Escherichia coli cells with a recN mutation did not exhibit increased nucleoid condensation, but chromosome replication and cytokinesis occurred. In the uurA and red strains, the condensation step was delayed compared to the wild-type strain, and chromosome replication and cytokinesis did not occur. The results are discussed with an emphasis on the functions of RecN, UvrA and Rec) in nucleoid reorganization in UV-irradiated E. coli cells.