Understanding the role of transcription in organisation of the bacterial chromosome

Abstract

Bacterial chromosomes are organised by various proteins, types of supercoiling and other cellular processes. One such process, transcription, massively impacts the chromosomal structure from the local level up to overall organisation of the nucleoid. Uniquely to bacterial transcription, the process can be physically coupled with translation since they occur in the same cellular compartment. The processes and associated proteins can, thus, happen simultaneously and physically interact. This coupling can have further impact of the overall structure of the nucleoid. While transcription-translation coupling is well documented in E. coli, some work suggests that it does not happen in other bacteria, including the Gram-positive model bacterium Bacillus subtilis. To study transcription-dependent chromosome organisation at a single cell level in B. subtilis, I fluorescently labelled DNA in the vicinity of the promoter of an inducible gene coding for a transmembrane protein and followed the localisation of the gene locus using fluorescence microscopy. We found that, upon induction, the gene migrates from a central position in the cell towards the membrane, and back towards the nucleoid when induction is removed. This movement was further confirmed by monitoring the fluorescently labelled locus in vertically immobilised cells (Vertical Cell Imaging by Nanostructured Immobilisation), which provides a better optical viewing angle for the observed process. Inhibiting either transcription and translation, via antibiotics and mutations in respective initiation regions, abolished the movement of fluorescently labelled locus towards the cell periphery. This loss of gene movement indicates the involvement of both transcription and translation in the process. Our results are fully consistent with transertion; a postulated process in which transmembrane proteins are inserted in the membrane co-translationally and cotranscriptionally thereby pulling the gene locus from the nucleoid core to the periphery of the cell and provide the first direct experimental evidence for transertion in Gram-positive bacteria. Furthermore, these findings demonstrate that translation and transcription can indeed be coupled in B. subtilis, alongside translocation, at least for genes encoding for membrane proteins.