Escherichia coli HU protein suppresses DNA-gyrase-mediated illegitimate recombination and SOS induction.

The HU protein is an abundant DNA binding protein of bacteria and is a major constituent of the bacterial nucleoid. HU protein is known to be involved in several fundamental biological functions, including DNA supercoiling, DNA replication, site-specific DNA inversion, and transposition. It is generally thought that a functional relationship exists between HU protein and DNA gyrase. We found that an hupA hupB double mutant displays enhanced spontaneous illegitimate recombination during the formation of lambdabio transducing phage in Escherichia coli. Nucleotide sequence analysis of the resulting transducing phages showed that the E. coli bio and lambda recombination sites did not have any homologous sequence. This mutation also enhanced the spontaneous expression of SOS functions. Furthermore, either overproduced GyrA protein or a temperature-sensitive gyrB mutation suppressed the illegitimate recombination enhanced by the defect of HU protein. These results show that the defect of HU induces illegitimate recombination and SOS response, which are probably mediated by DNA gyrase, implying that HU protein plays roles in suppression of illegitimate recombination and SOS response through interaction with DNA gyrase.