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A Gyrase Mutant with Low Activity Disrupts Supercoiling at the Replication Terminus

Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, Alabama 35294

Received 3 December 2004/ Accepted 22 August 2005

When a mutation in an essential gene shows a temperature-sensitive phenotype, one usually assumes that the protein is inactive at nonpermissive temperature. DNA gyrase is an essential bacterial enzyme composed of two subunits, GyrA and GyrB. The gyrB652 mutation results from a single base change that substitutes a serine residue for arginine 436 (R436-S) in the GyrB protein. At 42°C, strains with the gyrB652 allele stop DNA replication, and at 37°C, such strains grow but have RecA-dependent SOS induction and show constitutive RecBCD-dependent DNA degradation. Surprisingly, the GyrB652 protein is not inactive at 42°C in vivo or in vitro and it doesn't directly produce breaks in chromosomal DNA. Rather, this mutant has a low k_cat compared to wild-type GyrB subunit. With more than twice the normal mean number of supercoil domains, this gyrase hypomorph is prone to fork collapse and topological chaos near the terminus of DNA replication.

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