Identification of a Novel Streptococcal Gene Cassette Mediating SOS-mutagenesis in Streptococcus uberis

University of Helsinki, Department of Basic Veterinary Sciences, P.O. Box 66, FIN-00014, Finland; University of Helsinki, Institute of Biotechnology, P.O. Box 56, FIN-00014, Finland; National Public Health Institute, P.O. Box 57, 20521 Turku, Finland; University of Groningen, Department of Molecular Genetics, P.O. Box 14, 9750 AA Haren, the Netherlands

^* To whom correspondence should be addressed. Email: pekka.varmanen{at}helsinki.fi.

	Abstract

Streptococci have been considered to lack the classical SOS response, defined by increased mutation after UV exposure and regulation by LexA. Here we report identification of a potential self-regulated SOS mutagenesis gene cassette in the Streptococcaceae family. Exposure to UV-light was found to increase mutations to antibiotic resistance in Streptococcus uberis cultures. The mutational spectra revealed mainly G:CA:T transitions and Northern analyses demonstrated increased expression of a Y-family DNA polymerase resembling UmuC under DNA-damaging conditions. In the absence of the Y-family polymerase, S. uberis cells were sensitive to UV-light and to mitomycin C. Furthermore, the UV-induced mutagenesis was almost completely abolished in cells deficient in the Y-family polymerase. The gene encoding the the Y-family polymerase was localized in a four gene operon including two hypothetical genes and a gene encoding a HdiR homolog. Electrophoretic mobility shift assays demonstrated that S. uberis HdiR binds specifically to an inverted repeat sequence in the promoter region of the four gene operon. Data base searches revealed conservation of the gene cassette in several Streptococcus species including at least one genome of Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus mitis, Streptococcus sanguinis, and Streptococcus thermophilus strains. In addition, the umuC-operon was localized in several mobile DNA elements of Streptococcus and Lactococcus species. We conclude that the hdiR-umuC-ORF3-ORF4 operon represents a novel gene cassette capable of mediating of SOS mutagenesis among members of Streptococcaceae.