β-Glucosidase as a reporter for the gene expression studies in Thermus thermophilus and constitutive expression of DNA repair genes

摘要

Thermus thermophilus is an extremely thermophilic eubacterium that grows optimally at 70–75°C. Because the frequency of DNA damage, such as deamination, depurination and single-strand breaks, increases as the temperature rises, the regulation of expression as well as the specificities and activities of T.thermophilus DNA repair systems are of particular interest. To study those systems, we developed a gene expression vector using the T.thermophilus β-glucosidase gene (bgl) with host strain JOS9 (Δbgl) derived from the T.thermophilus wild-type strain HB27. Since HB27 has two putative β-galactosidase genes, the use of a single bgl gene as a reporter in combination with a Δbgl host strain permits the study of gene expression against a low background level. We assayed Bgl activity with 2-nitrophenyl-β-d-glucopyranoside as the substrate at 80°C. We measured the expression of seven genes involved in DNA repair—three nucleotide excision repair genes (uvrA, uvrB and uvrC) and four recombinational repair genes (recA, ruvA, ruvB and ruvC). Expression levels of uvrA and uvrB were about three times those of uvrC, while those of ruvA, ruvB and ruvC were almost equal. Both ruvA and ruvC formed an operon with their adjacent 5′-upstream gene paaG and ftsQAZ, respectively. recA was transcribed as an operon of four genes, amt-cinA-ligT-recA. All seven DNA repair genes were expressed constitutively, and the DNA damaging agent mitomycin C did not increase their expression.