Identification of DNA gyrase inhibitor (GyrI) in Escherichia coli

DNA gyrase is an essential enzyme in DNA replication in Escherichia coli. It mediates the introduction of negative supercoils near oriC, removal of positive supercoils ahead of the growing DNA fork, and separation of the two daughter duplexes. In the course of purifying DNA gyrase from E. coli KL16, we found an 18-kDa protein that inhibited the supercoiling activity of DNA gyrase, and we coined it DNA gyrase inhibitory 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. U00009) 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 yeeB and sbmC, we cloned the gene after amplification by polymerase chain reaction and purified the 18-kDa protein from an E. coli strain overexpressing it. The purified 18-kDa protein was confirmed to inhibit the supercoiling activity of DNA gyrase in vitro. In vivo, both overexpression and antisense expression of the gyrI gene induced filamentous growth of cells and suppressed cell proliferation. GyrI protein is the first identified chromosomally nucleoid-encoded regulatory factor of DNA gyrase in E. coli.