Escherichia coli strains lacking protein HU are UV sensitive due to a role for HU in homologous recombination

hupA and hupB encode the alpha and beta 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 of ruvA strains to a much lesser extent but enhance that of rus-1 ruvA strains to the same extent as for rus(+) 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 of hup 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.