Histone-like protein HU is required for recA gene-dependent DNA repair andSOS induction pathways in UV-irradiated Escherichia coli

Purpose: Escherichia coli HU protein exists as a heterodimer composed of tw o highly homologous subunits, HU-1 and HU-2, encoded by the hupB and hupA g enes, respectively. It introduces negative supercoils into a relaxed circul ar DNA. Various roles of HU have been suggested in cellular processes such as DNA replication and transcription. The present experiments were designed to understand the role of HU in DNA repair processes in E. coli. Materials and methods: The sensitivity of hupAhupB mutants of E. roll to th e lethal and mutagenic effects of UV was compared with that of a wild-type strain. The effect of the hupAhupB mutations in SOS induction was also exam ined. Results: The hupAhupB mutations increased the UV sensitivity of E. coli. Nu cleotide excision repair was unaffected by the deficiency of HU. On the oth er hand, E. roll hupAhupB mutants were sensitive to UV in the recA(+) recB( +) recF background but not in the recArecB(+)recF(+) or rerA(+)recBrecF(+) background. The frequency of UV-induced mutation to rifampicin resistance w as significantly reduced in the hupAhupB mutants, and the induction of the recA::lacZ and umuC::lacZ fusion genes was also suppressed in the mutants. Conclusions: HU protein plays a critical role in the recA, recB-dependent r ecombinational DNA repair and SOS induction pathways in UV-irradiated E. co li.