Syvertsson, S. | Search bioRxiv

bioRxiv2024

Divisome minimization shows that FtsZ and SepF can form an active Z-ring, and reveals BraB as a new cell division influencing protein in Bacillus subtilis

One of the aims of synthetic biology is the construction of a minimal cell. Since bacteria are the simplest life forms, they are the preferred blueprint for such a cell. In this study, we tried to find the minimal set of proteins required for division of a cell wall containing cell, using the bacterial model system Bacillus subtilis. Bacterial cytokinesis begins with polymerization of the tubulin homologue FtsZ at midcell into the Z-ring, which recruits the late cell division protein that synthesize the division septum. Assembly of FtsZ is carefully regulated, involving a dozen conserved cell division proteins. These proteins are not essential, but removing more than one is in many cases lethal. We made use of known suppressor mutations to find a gene deletion route that eventually enabled us to remove eight conserved cell division proteins: ZapA, MinC, MinJ, UgtP, ClpX, Noc, EzrA and FtsA. Only FtsZ and its membrane anchor SepF appeared to be required for Z-ring formation. Interestingly, SepF is also the FtsZ anchor in archaea, and both proteins may date back to the Last Universal Common Ancestor (LUCA). Curiously, viability was not greatly affected by the multiple deletions, although the frequency of cell division was considerably reduced. However, genome sequencing exposed the accumulation of several suppressor mutations, and revealed an unexpected cell division regulation function for the branched chain amino acid transporter BraB. The implications of these findings for the role of SepF in cell division, and the construction of a minimal cell division machinery are discussed.

microbiology↗

bioRxiv2021

Different resource allocation in a Bacillus subtilis population displaying bimodal motility

To cope with sudden changes in their environment, bacteria can use a bet-hedging strategy by dividing the population into cells with different properties. This so-called bimodal or bistable cellular differentiation is generally controlled by positive feedback regulation of transcriptional activators. Due to the continuous increase in cell volume, it is difficult for these activators to reach an activation threshold concentration when cells are growing exponentially. This is one reason why bimodal differentiation is primarily observed from the onset of the stationary phase when exponential growth ceases. An exception is the bimodal induction of motility in Bacillus subtilis, which occurs early during exponential growth. Several mechanisms have been put forward to explain this, including double negative-feedback regulation and the stability of the mRNA molecules involved. In this study, we used fluorescence-assisted cell sorting to compare the transcriptome of motile and non-motile cells and noted that expression of ribosomal genes is lower in motile cells. This was confirmed using an unstable GFP reporter fused to the strong ribosomal rpsD promoter. We propose that the reduction in ribosomal gene expression in motile cells is the result of a diversion of cellular resources to the synthesis of the chemotaxis and motility systems. In agreement, single-cell microscopic analysis showed that motile cells are slightly shorter than non-motile cells, an indication of slower growth. We speculate that this growth rate reduction can contribute to the bimodal induction of motility during exponential growth. IMPORTANCETo cope with sudden environmental changes, bacteria can use a bet-hedging strategy and generate different types of cells within a population, so called bimodal differentiation. For example, a Bacillus subtilis culture can contain both motile and non-motile cells. In this study we compared the gene expression between motile and non-motile cells. It appeared that motile cells express less ribosomes. To confirm this, we constructed a ribosomal promoter fusion that enabled us to measure expression of this promoter in individual cells. This reporter fusion confirmed our initial finding. The re-allocation of cellular resources from ribosome synthesis towards synthesis of the motility apparatus results in a reduction in growth. Interestingly, this growth reduction has been shown to stimulate bimodal differentiation.

microbiology↗