Analysis of transcription at the oriC locus in Mycobacterium tuberculosis

Assessment of closely related Mycobacterium tuberculosis variants with different transmission success and in vitro infection dynamics

Whole genome sequencing (WGS) is able to differentiate closely related Mycobacterium tuberculosis variants within the same transmission cluster. Our aim was to evaluate if this higher discriminatory power may help identify and characterize more actively transmitted variants and understand the factors behind their success. We selected a robust MIRU-VNTR-defined cluster from Almería, Spain (22 cases throughout 2003–2019). WGS allowed discriminating, within the same epidemiological setting, between a successfully transmitted variant and seven closely related variants that did not lead to secondary cases, or were involved in self-limiting transmission (one single secondary case). Intramacrophagic growth of representative variants was evaluated in an in vitro infection model using U937 cells. Intramacrophage multiplication ratios (CFUs at Day 4/CFUs at Day 0) were higher for the actively transmitted variant (range 5.3–10.7) than for the unsuccessfully transmitted closely related variants (1.5–3.95). Two SNPs, mapping at the DNA binding domain of DnaA and at kdpD, were found to be specific of the successful variant.

Beyond DnaA: the role of DNA topology and DNA methylation in bacterial replication initiation

The replication of chromosomal DNA is a fundamental event in the life cycle of every cell. The first step of replication, initiation, is controlled by multiple factors to ensure only one round of replication per cell cycle. The process of initiation has been described most thoroughly for bacteria, especially Escherichia coli, and involves many regulatory proteins that vary considerably between different species. These proteins control the activity of the two key players of initiation in bacteria: the initiator protein DnaA and the origin of chromosome replication (oriC). Factors involved in the control of the availability, activity, or oligomerization of DnaA during initiation are generally regarded as the most important and thus have been thoroughly characterized. Other aspects of the initiation process, such as origin accessibility and susceptibility to unwinding, have been less explored. However, recent findings indicate that these factors have a significant role. This review focuses on DNA topology, conformation, and methylation as important factors that regulate the initiation process in bacteria. We present a comprehensive summary of the factors involved in the modulation of DNA topology, both locally at oriC and more globally at the level of the entire chromosome. We show clearly that the conformation of oriC dynamically changes, and control of this conformation constitutes another, important factor in the regulation of bacterial replication initiation. Furthermore, the process of initiation appears to be associated with the dynamics of the entire chromosome and this association is an important but largely unexplored phenomenon.

Characterization of microevolution events in Mycobacterium tuberculosis strains involved in recent transmission clusters

Under certain circumstances, it is possible to identify clonal variants of Mycobacterium tuberculosis infecting a single patient, probably as a result of subtle genetic rearrangements in part of the bacillary population. We systematically searched for these microevolution events in a different context, namely, recent transmission chains. We studied the clustered cases identified using a population-based universal molecular epidemiology strategy over a 5-year period. Clonal variants of the reference strain defining the cluster were found in 9 (12%) of the 74 clusters identified after the genotyping of 612 M. tuberculosis isolates by IS6110 restriction fragment length polymorphism analysis and mycobacterial interspersed repetitive units-variable-number tandem repeat typing. Clusters with microevolution events were epidemiologically supported and involved 4 to 9 cases diagnosed over a 1- to 5-year period. The IS6110 insertion sites from 16 representative isolates of reference and microevolved variants were mapped by ligation-mediated PCR in order to characterize the genetic background involved in microevolution. Both intragenic and intergenic IS6110 locations resulted from these microevolution events. Among those cases of IS6110 locations in intergenic regions which could have an effect on the regulation of adjacent genes, we identified the overexpression of cytochrome P450 in one microevolved variant using quantitative real-time reverse transcription-PCR. Our results help to define the frequency with which microevolution can be expected in M. tuberculosis transmission chains. They provide a snapshot of the genetic background of these subtle rearrangements and identify an event in which IS6110-mediated microevolution in an isogenic background has functional consequences.