Abstract
Filamentous bacteria of the genus Streptomyces possess linear chromosomes and linear plasmids. Theoretically, linear replicons may not need a decatenase for post-replicational separation of daughter molecules. Yet, Streptomyces contain parC and parE that encode the subunits for the decatenase topoisomerase IV. The linear replicons of Streptomyces adopt a circular configuration in vivo through telomere–telomere interaction, which would require decatenation, if the circular configuration persists through replication. We investigated whether topoisomerase IV is required for separation of the linear replicons in Streptomyces. Deletion of parE from the Streptomyces coelicolor chromosome was achieved, when parE was provided on a plasmid. Subsequently, the plasmid was eliminated at high temperature, and ΔparE mutants were obtained. These results indicated that topoisomerase IV was not essential for Streptomyces. Presumably, the telomere–telomere association may be resolved during or after replication to separate the daughter chromosomes. Nevertheless, the mutants exhibited retarded growth, defective sporulation and temperature sensitivity. In the mutants, circular plasmids could not replicate, and spontaneous circularization of the chromosome was not observed, indicating that topoisomerase IV was required for decatenation of circular replicons. Moreover, site-specific integration of a plasmid is impaired in the mutants, suggesting the formation of DNA knots during integration, which must be resolved by topoisomerase IV.
Highlights
Most bacterial chromosomes consist of covalently closed circular DNA with negative superhelicity
Our results showed that deletion of parE could be achieved on a linear chromosome but not on a circular chromosome, indicating that topoisomerase IV (Topo IV) was essential for circular DNA but not for linear DNA in Streptomyces
Topo IV is essential for segregation of circular daughter chromosomes and plasmids in bacteria
Summary
Most bacterial chromosomes consist of covalently closed circular DNA with negative superhelicity. The gyrase–Topo I pair acts in concert to relieve the superhelicity generated during replication and transcription, i.e. the local positive supercoiling ahead of the replication forks and transcription bubbles is relaxed by gyrase, and the local negative supercoiling behind the transcription bubbles is compensated by Topo I. Because of these important physiological roles, gyrase and Topo I are basically essential for viability of bacterial cells, some defects in one of these proteins may be tolerable or suppressed by mutation in the other
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