MreB: pilot or passenger of cell wall synthesis?

The discovery that the bacterial cell shape determinant MreB is related to actin spurred new insights into bacterial morphogenesis and development. The trafficking and mechanical roles of the eukaryotic cytoskeleton were hypothesized to have a functional ancestor in MreB based on evidence implicating MreB as an organizer of cell wall synthesis. Genetic, biochemical and cytological studies implicate MreB as a coordinator of a large multi-protein peptidoglycan (PG) synthesizing holoenzyme.

Direct interaction of FliX and FlbD is required for their regulatory activity in Caulobacter crescentus.

Background: The temporal and spatial expression of late flagellar genes in Caulobacter crescentus is activated by the transcription factor FlbD and its partner trans-acting factor FliX. The physical interaction of these two proteins represents an alternative mechanism for regulating the activity of σ54 transcription factors. This study is to characterize the interaction of the two proteins and the consequences of the interaction on their regulatory activity.

Positioning cell wall synthetic complexes by the bacterial morphogenetic proteins MreB and MreD.

In Caulobacter crescentus, intact cables of the actin homologue, MreB, are required for the proper spatial positioning of MurG which catalyses the final step in peptidoglycan precursor synthesis. Similarly, in the periplasm, MreC controls the spatial orientation of the penicillin binding proteins and a lytic transglycosylase. We have now found that MreB cables are required for the organization of several other cytosolic murein biosynthetic enzymes such as MraY, MurB, MurC, MurE and MurF.

Global regulation of gene expression and cell differentiation in Caulobacter crescentus in response to nutrient availability.

In a developmental strategy designed to efficiently exploit and colonize sparse oligotrophic environments, Caulobacter crescentus cells divide asymmetrically, yielding a motile swarmer cell and a sessile stalked cell. After a relatively fixed time period under typical culture conditions, the swarmer cell differentiates into a replicative stalked cell. Since differentiation into the stalked cell type is irreversible, it is likely that environmental factors such as the availability of essential nutrients would influence the timing of the decision to abandon motility and adopt a sessile lifestyle.

The cell shape proteins MreB and MreC control cell morphogenesis by positioning cell wall synthetic complexes.

MreB, the bacterial actin homologue, is thought to function in spatially co-ordinating cell morphogenesis in conjunction with MreC, a protein that wraps around the outside of the cell within the periplasmic space. In Caulobacter crescentus, MreC physically associates with penicillin-binding proteins (PBPs) which catalyse the insertion of intracellularly synthesized precursors into the peptidoglycan cell wall. Here we show that MreC is required for the spatial organization of components of the peptidoglycan-synthesizing holoenzyme in the periplasm and MreB directs the localization of a peptidoglycan precursor synthesis protein in the cytosol.

The cell-shape protein MreC interacts with extracytoplasmic proteins including cell wall assembly complexes in Caulobacter crescentus.

The bacterial actin homolog, MreB, forms helical cables within the cell that are required for maintenance of a rod shape. These helical structures are thought to be involved in the spatial organization of cell wall (peptidoglycan) synthesizing complexes of penicillin-binding proteins (PBPs). Here, we examined the role of the MreC cell shape protein in this process in Caulobacter crescentus.

The trans-acting flagellar regulatory proteins, FliX and FlbD, play a central role in linking flagellar biogenesis and cytokinesis in Caulobacter crescentus.

The FliX/FlbD-dependent temporal transcription of late flagellar genes in Caulobacter crescentus requires the assembly of an early, class II-encoded flagellar structure. Class II flagellar-mutant strains exhibit a delay in the completion of cell division, with the accumulation of filamentous cells in culture. It is shown here that this cell-division defect is attributable to an arrest in the final stages of cell separation.

Linking structural assembly to gene expression: a novel mechanism for regulating the activity of a sigma54 transcription factor.

In Caulobacter crescentus, the temporal and spatial expression of late flagellar genes is regulated by the sigma54 transcriptional activator, FlbD. Genetic experiments have indicated that the trans-acting factor FliX regulates FlbD in response to the progression of flagellar assembly, repressing FlbD activity until an early flagellar basal body structure is assembled. Following assembly of this structure, FliX is thought to function as an activator of FlbD.

The conserved flaF gene has a critical role in coupling flagellin translation and assembly in Caulobacter crescentus.

The expression of the flagellin proteins in Caulobacter crescentus is regulated by the progression of flagellar assembly both at the transcriptional and post-transcriptional levels. An early basal body structure is required for the transcription of flagellin genes, whereas the ensuing assembly of a hook structure is required for flagellin protein synthesis. Previous experiments have shown that the negative regulatory protein, FlbT, operates this second post-transcriptional checkpoint by associating with the 5' untranslated region (UTR) of the fljK flagellin transcript, inhibiting translation and destabilizing the mRNA.

Role of core promoter sequences in the mechanism of swarmer cell-specific silencing of gyrB transcription in Caulobacter crescentus.

Background: Each Caulobacter crescentus cell division yields two distinct cell types: a flagellated swarmer cell and a non-motile stalked cell. The swarmer cell is further distinguished from the stalked cell by an inability to reinitiate DNA replication, by the physical properties of its nucleoid, and its discrete program of gene expression. Specifically, with regard to the latter feature, many of the genes involved in DNA replication are not transcribed in swarmer cells.

Role of integration host factor in the transcriptional activation of flagellar gene expression in Caulobacter crescentus.

In the Caulobacter crescentus predivisional cell, class III and IV flagellar genes, encoding the extracytoplasmic components of the flagellum, are transcribed in the nascent swarmer compartment. This asymmetric expression pattern is attributable to the compartmentalized activity of the sigma54-dependent transcriptional activator FlbD. Additionally, these temporally transcribed flagellar promoters possess a consensus sequence for the DNA-binding protein integration host factor (IHF), located between the upstream FlbD binding site and the promoter sequences.

Regulation of FlbD activity by flagellum assembly is accomplished through direct interaction with the trans-acting factor, FliX.

The temporal and spatial transcription of late flagellar genes in Caulobacter crescentus is regulated by the sigma54 transcriptional activator, FlbD. One requirement for FlbD activity is the assembly of a structure encoded by early, class II flagellar genes. In this report, we show that the trans-acting factor FliX predominantly functions as a negative regulator of FlbD activity in the absence of the class II-encoded flagellar structure.

Measurements of DNA lengths remaining in a viral capsid after osmotically suppressed partial ejection.

The effect of external osmotic pressure on the extent of DNA ejection from bacteriophage-lambda was recently investigated (Evilevitch et al., 2003). The total length of DNA ejected was measured via the 260-nm absorption by free nucleotides, after opening of the capsids in the presence of varying amounts of polyethylene glycol 8000 and DNase I.

MreB, the cell shape-determining bacterial actin homologue, co-ordinates cell wall morphogenesis in Caulobacter crescentus.

The bacterial actin homologue, MreB, is required for the maintenance of a rod-shaped cell and has been shown to form spirals that traverse along the longitudinal axis of Bacillus subtilis and Escherichia coli cells. The depletion of MreB in Caulobacter crescentus resulted in lemon-shaped cells that possessed defects in the integrity of the cell wall. MreB localization appeared as bands or spirals that encircled the cell along its entire length and switched to a mid-cell location at a time that coincided with the initiation of cell division.

Cell shape, division and development: the 2002 American Society for Microbiology (ASM) conference on prokaryotic development.

In the last decade, the use of cytological techniques, together with the analysis of complete genomes, has dramatically advanced our understanding of bacterial development. Work on several well-developed model systems such as Bacillus subtilis, Caulobacter crescentus, Myxococcus xanthus and Streptomyces spp., has provided us with an in-depth understanding of processes such as sporulation, multicellular behaviour and the bacterial cell cycle.

Productive interaction between the chromosome partitioning proteins, ParA and ParB, is required for the progression of the cell cycle in Caulobacter crescentus.

In Caulobacter crescentus the partitioning proteins ParA and ParB operate a molecular switch that couples chromosome partitioning to cytokinesis. Homologues of these proteins have been shown to be important for the stable inheritance of F-plasmids and the prophage form of bacteriophage P1. In C.

ParB-stimulated nucleotide exchange regulates a switch in functionally distinct ParA activities.

ParA and ParB of Caulobacter crescentus belong to a conserved family of bacterial proteins implicated in chromosome segregation. ParB binds to DNA sequences adjacent to the origin of replication and localizes to opposite cell poles shortly following the initiation of DNA replication. ParA has homology to a conserved and widespread family of ATPases.

Bacterial chromosome segregation.

Recent studies have made great strides toward our understanding of the mechanisms of microbial chromosome segregation and partitioning. This review first describes the mechanisms that function to segregate newly replicated chromosomes, generating daughter molecules that are viable substrates for partitioning. Then experiments that address the mechanisms of bulk chromosome movement are summarized.

Mutations in FlbD that relieve the dependency on flagellum assembly alter the temporal and spatial pattern of developmental transcription in Caulobacter crescentus.

The transcription factor FlbD regulates the temporal and spatial transcription of flagellar genes in the bacterium Caulobacter crescentus. Activation of FlbD requires cell cycle progression and the assembly of an early (class II) flagellum structure. In this report, we identify 20 independent gain-of-function mutations in flbD that relieve regulation by flagellar assembly.