Genes and the replication cycle of Escherichia coli

Morphogenesis of Escherichia coli

The shape of Escherichia coli is strikingly simple compared to those of higher eukaryotes. In fact, the end result of E. coli morphogenesis is a cylindrical tube with hemispherical caps. It is argued that physical principles affect biological forms. In this view, genes code for products that contribute to the production of suitable structures for physical factors to act upon. After introduction of a physical model, the discussion is focused on the shape-maintaining (peptidoglycan) layer of E. coli. This is followed by a detailed analysis of the structural relationship of the cellular interior to the cytoplasmic membrane. A basic theme of this review is that the transcriptionally active nucleoid and the cytoplasmic translation machinery form a structural continuity with the growing cellular envelope. An attempt has been made to show how this dynamic relationship during the cell cycle affects cell polarity and how it leads to cell division.

An M. tuberculosis DNA fragment contains genes encoding cell division proteins ftsX and ftsE, a basic protein and homologues of PemK and small protein B

A 4-kb fragment of the M. tuberculosis chromosome was identified which contains several genes including those involved in cell division and possibly macrophage survival. DNA sequence analysis revealed open reading frames (ORFs) encoding putative proteins bearing significant homology with proteins FtsX and FtsE associated with cell division in E. coli, with PemK protein which inhibits cell division in E. coli harboring plasmid R100 and with SmpB protein of Salmonella typhimurium implicated in its survival within macrophages. The ftsX gene is conserved among mycobacteria belonging to the M. tuberculosis Complex. Furthermore, ftsX-specific transcripts were prevalent in equivalent amounts in M. tuberculosis H37Rv and H37Ra as analyzed by RT-PCR and primer extension. Transcription start points (tsp) a and b map in the region upstream of the FtsX ORF whose promoter activity was established by (i) a promoter-fusion experiment and (ii) by mapping the 5' ends of transcripts derived from the promoter-fusion construct. FtsX transcription is modulated as a function of mycobacterial growth and division status, maximum expression being observed in log phase cells. Growth-related expression of ftsX may provide a basis for developing a marker to distinguish actively replicating M. tuberculosis cells from quiescent mycobacteria.

Development of the cell-division site in FtsA- filaments

Early changes at cell-division sites were studied in non-septate filaments induced by growth of ftsATs mutant cells under non-permissive conditions. The positions of localized regions of plasmolysis were used as markers for the locations of partial and complete annular structures that are thought to be precursors of the periseptal annuli that flank the septum during cytokinesis. The results confirmed that these structures were localized at potential division sites and suggested a model in which older division sites play a role in the generation of new sites for future use, with each older site being used only once for this purpose. The results also suggest that the details of division-site development can profitably be studied in cells in which early events in the differentiation process are uncoupled from the septation event.

The Bacillus subtilis spoVD gene encodes a mother-cell-specific penicillin-binding protein required for spore morphogenesis

The Bacillus subtilis spoVD gene has been cloned and sequenced. It encodes a 71,262 Da protein with extensive sequence similarity to penicillin-binding proteins from various organisms. The context of this gene in the B. subtilis chromosome, immediately upstream of the mur operon, suggests that it is related to the pbpB gene of Escherichia coli, which is involved in the synthesis of septal peptidoglycan during cell division. Expression of spoVD in E. coli leads to the synthesis of a membrane-associated protein of the size expected for SpoVD, which can bind labelled penicillin. However, insertional disruption of the spoVD gene has no effect on vegetative growth or division: a second pbp-like gene immediately upstream of spoVD is probably the functional homologue of E. coli pbpB. spoVD seems instead to have a specialized role in the morphogenesis of the spore cortex, which is a modified form of peptidoglycan. spoVD transcription appears to occur from a promoter recognized by the sigma E form of RNA polymerase. Analysis of the expression of a spoVD'-lacZ reporter gene supports this notion and indicates that a second level of negative regulation is dependent on the SpoIIID protein. SpoVD synthesis probably occurs only in the mother cell since both sigma E and SpoIIID are thought to be specific to this cell type. Such localization of SpoVD synthesis was supported by the results of a genetic test showing that expression of spoVD only in the mother cell is sufficient for spore formation. The results support the proposition that spore cortex formation is determined primarily by the mother cell.

Chromosome partition in Escherichia coli

The past year has seen important genetic and biochemical advances in our understanding of the mechanisms that are involved in chromosome partition into two daughter cells in Escherichia coli. Topoisomerase IV and XerCD recombinase have been shown to be required for the unlinking of replicated chromosomes. MukB, an alpha-helical coiled-coil protein, has been shown to be involved in chromosome partition, and this is the first candidate for a bacterial motor protein. Another protein, FtsZ, has been shown to form a constriction ring in cell division and may also relate to chromosome partition.

Microcin 25, a novel antimicrobial peptide produced by Escherichia coli

Microcin 25, a peptide antibiotic excreted by an Escherichia coli strain isolated from human feces, was purified to homogeneity and characterized. Composition analysis and data from gel filtration indicated that microcin 25 may contain 20 amino acid residues. It has a blocked amino-terminal end. Microcin synthesis and immunity are plasmid determined, and the antibiotic was produced in minimal medium when the cultures entered the stationary phase of growth. The peptide appears to interfere with cell division, since susceptible cells filamented when exposed to it. This response does not seem to be mediated by the SOS system.

Creative blocks: cell-cycle checkpoints and feedback controls

Before division, cells must ensure that they finish DNA replication, DNA repair and chromosome segregation. They do so by using feedback controls which can detect the failure to complete replication, repair or spindle assembly to arrest the progress of the cell cycle at one of three checkpoints. Failures in feedback controls can contribute to the generation of cancer.

What is the minimum number of dedicated functions required for a basic cell cycle?

The genome of Escherichia coli has a coding capacity for about 4500 proteins but only a small number of these appear to be specific for the periodic events (initiation of DNA replication, chromosome partitioning and cell division) that punctuate the cell-duplication cycle: furthermore, many of these cell cycle dedicated functions are dispensible under certain conditions, although their presence undoubtedly increases the fitness of the organism to survive in a competitive environment. A simplified but effective cell replication cycle can probably operate with only a few cycle-dedicated proteins, in addition to those required for cell growth itself.

Localised mutagenesis of the fts YEX operon: conditionally lethal missense substitutions in the FtsE cell division protein of Escherichia coli are similar to those found in the cystic fibrosis transmembrane conductance regulator protein (CFTR) of human patients

After localised mutagenesis of the 76 min region of the Escherichia coli chromosome, we isolated a number of conditionally lethal mutants. Some of these mutants had a filamentation temperature sensitive (fts) phenotype and were assigned to the cell division genes ftsE of ftsX, whereas others were defective in the heat shock regulator gene rpoH. Both missense and amber mutant alleles of these genes were produced. The missense mutant ftsE alleles were cloned and sequenced to determine whether or not the respective mutations mapped to the region of the gene encoding the putative nucleotide binding site. Surprisingly, most of these mutant FtsE proteins had missense substitutions in a different domain of the protein. This region of the FtsE protein is highly conserved in a large family of proteins involved in diverse transport processes in all living cells, from bacteria to man. One of the proteins in this large family of homologues is the human cystic fibrosis transmembrane conductance regulator (CFTR), and the FtsE substitutions were found to be in very closely linked, or identical, amino acid residues to those which are frequently altered in the CFTR of human patients. These results confirm the structural importance of this highly conserved region of FtsE and CFTR and add weight to the current structural model for the human protein.

Cell division and peptidoglycan assembly in Escherichia coli

Research on bacterial cell division has recently gained renewed impetus because of new information about peptidoglycan assembly and about specific cell-division genes and their products. This paper concerns aspects of cell division that specifically concern the peptidoglycan. It is shown that upon division, peptidoglycan assembly switches from lateral wall location to the cell centre, that assembly takes place at the leading edge of the invaginating constriction, that the mode of glycan strand insertion changes from a single-stranded mode to a multi-stranded mode, and that the initiation of division (in contrast to its continuation) requires penicillin-insensitive peptidoglycan synthesis (PIPS). A membrane component X (possibly FtsQ) is proposed to coordinate PIPS with the cell division-initiating protein FtsZ. It is suggested that a largely proteinaceous macromolecular complex (divisome) at the leading edge of constriction encompasses three compartments (cytoplasm, membrane and periplasm). The composition of this complex is proposed to vary depending on whether division is being initiated or completed.

The balance between different peptidoglycan precursors determines whether Escherichia coli cells will elongate or divide

The rodA(Sui) mutation allows cell division to take place at 42 degrees C in ftsI23 mutant cells, which produce a thermolabile penicillin-binding protein 3 (PBP3, the septation-specific peptidoglycan transpeptidase). We show here that the mutation in rodA is a single-base change from a glutamine to a chain termination (amber) codon, and that an amber suppressor (supE) present in the strain restores the ability to produce a reduced level of normal RodA protein. The reduced level of RodA is accompanied by an increase in the levels of two other proteins (PBP2 and PBP5) encoded by genes in the rodA operon. We show that an increased level of PBP5 is by itself sufficient to restore cell division to ftsI23 cells at 42 degrees C. Two other treatments were found to restore division capacity to the mutant: an increase in PBP6 (which is a D-alanine carboxypeptidase like PBP5) or suitable concentrations of D-cycloserine. All of the above treatments have the effect of reducing the number of pentapeptide side chains in peptidoglycan and increasing the number of tripeptides. We conclude that the effect of the rodA(Sui) mutation is to indirectly increase the availability of tripeptide side chains, which are used preferentially by PBP3 as acceptors in transpeptidation. A change in the proportions of different kinds of peptide side chain in the peptidoglycan can therefore determine whether cells will divide.