IDENTIFICATION OF DNA GYRASE INHIBITOR (GYRI) IN ESCHERICHIA-COLI

DNA gyrase is an essential enzyme in DNA replication in Escherichia co il. It mediates the introduction of negative supercoils near oriC, rem oval of positive supercoils ahead of the growing DNA fork, and separat ion of the two daughter duplexes, In the course of purifying DNA gyras e from E. coil KL16, we found an 18-kDa protein that inhibited the sup ercoiling activity of DNA gyrase, and we coined it DNA gyrase inhibito ry protein (GyrI). Its NH2-terminal amino acid sequence of 16 residues was determined to be identical to that of a putative gene product (a polypeptide of 157 amino acids) encoded by yeeB (EMBL accession no. U0 0009) and sbmC (Baquero, M. R., Bouzon, M., Varea, J., and Moreno, F. (1995) Mol. Microbiol. 18, 301-311) of E. coli. Assuming the identity of the gene (gyrI) encoding GyrI with the previously reported genes ye eB and sbmC, we cloned the gene after amplification by polymerase chai n reaction and purified the 18-kDa protein from an E. coil strain over expressing it. The purified 18-kDa protein was confirmed to inhibit th e supercoiling activity of DNA gyrase in vitro. In vivo, both overexpr ession and antisense expression of the gyrI gene induced filamentous g rowth of cells and suppressed cell proliferation, GyrI protein is the first identified chromosomally nucleoid-encoded regulatory factor of D NA gyrase in E. coli.