RESUMEN
The opposing catalytic activities of topoisomerase I (TopoI/relaxase) and DNA gyrase (supercoiling enzyme) ensure homeostatic maintenance of bacterial chromosome supercoiling. Earlier studies in Escherichia coli suggested that the alteration in DNA supercoiling affects the DNA gyrase and TopoI expression. Although, the role of DNA elements around the promoters were proposed in regulation of gyrase, the molecular mechanism of supercoiling mediated control of TopoI expression is not yet understood. Here, we describe the regulation of TopoI expression from Mycobacterium tuberculosis and Mycobacterium smegmatis by a mechanism termed Supercoiling Sensitive Transcription (SST). In both the organisms, topoI promoter(s) exhibited reduced activity in response to chromosome relaxation suggesting that SST is intrinsic to topoI promoter(s). We elucidate the role of promoter architecture and high transcriptional activity of upstream genes in topoI regulation. Analysis of the promoter(s) revealed the presence of sub-optimal spacing between the -35 and -10 elements, rendering them supercoiling sensitive. Accordingly, upon chromosome relaxation, RNA polymerase occupancy was decreased on the topoI promoter region implicating the role of DNA topology in SST of topoI. We propose that negative supercoiling induced DNA twisting/writhing align the -35 and -10 elements to facilitate the optimal transcription of topoI.
Asunto(s)
Girasa de ADN/biosíntesis , ADN-Topoisomerasas de Tipo I/biosíntesis , Homeostasis/genética , Transcripción Genética , Girasa de ADN/genética , ADN-Topoisomerasas de Tipo I/genética , ADN Superhelicoidal/genética , ARN Polimerasas Dirigidas por ADN/genética , Regulación Bacteriana de la Expresión Génica , Mycobacterium smegmatis/enzimología , Mycobacterium smegmatis/genética , Mycobacterium tuberculosis/enzimología , Mycobacterium tuberculosis/genética , Regiones Promotoras GenéticasRESUMEN
The steady-state negative supercoiling of eubacterial genomes is maintained by the action of DNA topoisomerases. Topoisomerase distribution varies in different species of mycobacteria. While Mycobacterium tuberculosis (Mtb) contains a single type I (TopoI) and a single type II (Gyrase) enzyme, Mycobacterium smegmatis (Msm) and other members harbour additional relaxases. TopoI is essential for Mtb survival. However, the necessity of TopoI or other relaxases in Msm has not been investigated. To recognize the importance of TopoI for growth, physiology and gene expression of Msm, we have developed a conditional knock-down strain of TopoI in Msm. The TopoI-depleted strain exhibited extremely slow growth and drastic changes in phenotypic characteristics. The cessation of growth indicates the essential requirement of the enzyme for the organism in spite of having additional DNA relaxation enzymes in the cell. Notably, the imbalance in TopoI level led to the altered expression of topology modulatory proteins, resulting in a diffused nucleoid architecture. Proteomic and transcript analysis of the mutant indicated reduced expression of the genes involved in central metabolic pathways and core DNA transaction processes. RNA polymerase (RNAP) distribution on the transcription units was affected in the TopoI-depleted cells, suggesting global alteration in transcription. The study thus highlights the essential requirement of TopoI in the maintenance of cellular phenotype, growth characteristics and gene expression in mycobacteria. A decrease in TopoI level led to altered RNAP occupancy and impaired transcription elongation, causing severe downstream effects.
Asunto(s)
Proteínas Bacterianas/metabolismo , Núcleo Celular/metabolismo , ADN-Topoisomerasas de Tipo I/metabolismo , Mycobacterium smegmatis/enzimología , Mycobacterium smegmatis/crecimiento & desarrollo , Proteínas Bacterianas/genética , Núcleo Celular/química , Núcleo Celular/genética , ADN-Topoisomerasas de Tipo I/genética , Regulación Bacteriana de la Expresión Génica , Mycobacterium smegmatis/química , Mycobacterium smegmatis/genética , FenotipoRESUMEN
Topoisomerases are an important class of enzymes for regulating the DNA transaction processes. Mycobacterium tuberculosis (Mtb) is one of the most formidable pathogens also posing serious challenges for therapeutic interventions. The organism contains only one type IA topoisomerase (Rv3646c), offering an opportunity to test its potential as a candidate drug target. To validate the essentiality of M. tuberculosis topoisomerase I (TopoI(Mt) ) for bacterial growth and survival, we have generated a conditionally regulated strain of topoI in Mtb. The conditional knockdown mutant exhibited delayed growth on agar plate. In liquid culture, the growth was drastically impaired when TopoI expression was suppressed. Additionally, novobiocin and isoniazid showed enhanced inhibitory potential against the conditional mutant. Analysis of the nucleoid revealed its altered architecture upon TopoI depletion. These studies establish the essentiality of TopoI for the M. tuberculosis growth and open up new avenues for targeting the enzyme.
