Escherichia coli Strains Lacking Protein HU Are UV Sensitive due to a Role for HU in Homologous Recombination
ABSTRACT hupA and hupB encode the α and β subunits of the Escherichia coli histone-like protein HU. Here we show that E. coli hup mutants are sensitive to UV in the rec+ sbc +, recBC sbcA, recBC sbcBC, umuDC,recF, and recD backgrounds. However,hupAB mutations do not enhance the UV sensitivity of resolvase-deficient recG ruvA strains. hupAB uvrA and hupAB recG strains are supersensitive to UV.hup mutations enhance the UV sensitivity ofruvA strains to a much lesser extent but enhance that ofrus-1 ruvA strains to the same extent as forrus+ ruv + strains. Our results suggest that HU plays a role in recombinational DNA repair that is not specifically limited to double-strand break repair or daughter strand gap repair; the lack of HU affects the RecG RusA and RuvABC pathways for Holliday junction processing equally if the two pathways are equally active in recombinational repair; the function of HU is not in the substrate processing step or in the RecFOR-directed synapsis action during recombinational repair. Furthermore, the UV sensitivity ofhup mutants cannot be suppressed by overexpression of wild-type or mutant gyrB, which confers novobiocin resistance, or by different concentrations of a gyrase inhibitor that can increase or decrease the supercoiling of chromosomal DNA.