ABSTRACT: Negative DNA supercoiling allows chromosome condensation and facilitates DNA unwinding, which is required for the occurrence of DNA transaction processes, i.e., DNA replication, transcription and recombination. In bacteria, changes in chromosome supercoiling impact global gene expression; however, the limited studies on the global transcriptional response have focused mostly on pathogenic species and have reported various fractions of affected genes. Furthermore, the transcriptional response to long-term supercoiling imbalance is still poorly understood. Here, we address the transcriptional response to both novobiocin-induced rapid chromosome relaxation or long-term topological imbalance, both increased and decreased supercoiling, in environmental antibiotic-producing bacteria belonging to the Streptomyces genus. During the Streptomyces complex developmental cycle, multiple copies of GC-rich linear chromosomes present in hyphal cells undergo profound topological changes, from being loosely condensed in vegetative hyphae, to being highly compacted in spores. Moreover, changes in chromosomal supercoiling have been suggested to be associated with the control of antibiotic production and environmental stress response. Remarkably, in S. coelicolor, a model Streptomyces species, topoisomerase I (TopA) is solely responsible for the removal of negative DNA supercoils. Using a S. coelicolor strain in which topA transcription is under the control of an inducible promoter, we identified genes involved in the transcriptional response to long-term supercoiling imbalance. The affected genes are preferentially organized in several clusters, and a supercoiling-hypersensitive cluster (SHC) was found to be located in the core of the S. coelicolor chromosome. The transcripts affected by long-term topological imbalance encompassed genes encoding nucleoid-associated proteins, DNA repair proteins and transcriptional regulators, including multiple developmental regulators. Moreover, using a gyrase inhibitor, we identified those genes that were directly affected by novobiocin, and found this was correlated with increased AT content in their promoter regions. In contrast to the genes affected by long-term supercoiling changes, among the novobiocin-sensitive genes, a significant fraction encoded for proteins associated with membrane transport or secondary metabolite synthesis. Collectively, our results show that long-term supercoiling imbalance globally regulates gene transcription and has the potential to impact development, secondary metabolism and DNA repair, amongst others.
