Regulation of cell division in E. coli

Recent investigation of some old cell division mutants of E. coli suggests that genes playing central roles in the regulation of division have been identified. The results suggest that cell division is triggered when a critical level of a single protein, FtsZ, is attained. The activity of this protein is channelled to the new division site by the activity of the min locus, which blocks access to old sites. Continued study of these genes should yield further insights into the cell division process.

Nucleotide sequence and insertional inactivation of a Bacillus subtilis gene that affects cell division, sporulation, and temperature sensitivity

Located at 135 degrees on the Bacillus subtilis genetic map are several genes suspected to be involved in cell division and sporulation. Previously isolated mutations mapping at 135 degrees include the tms-12 mutation and mutations in the B. subtilis homologs of the Escherichia coli cell division genes ftsA and ftsZ. Previously, we cloned and sequenced the B. subtilis ftsA and ftsZ genes that are present on an 11-kilobase-pair EcoRI fragment and found that the gene products and organization of these two genes are conserved between the two bacterial species. We have since found that the mutation in the temperature-sensitive filamenting tms-12 mutant maps upstream of the ftsA gene on the same 11-kilobase-pair EcoRI fragment in a gene we designated dds. Sequence analysis of the dds gene and four other open reading frames upstream of ftsA revealed no significant homology to other known genes. It was found that the dds gene is not absolutely essential for viability since the dds gene could be insertionally inactivated. The dds null mutants grew slowly, were filamentous, and exhibited a reduced level of sporulation. Additionally, these mutants were extremely temperature sensitive and were unable to form colonies at 37 degrees C. Another insertion, which resulted in the elimination of 103 C-terminal residues, resulted in a temperature-sensitive phenotype less severe than that in the dds null mutant and similar to that in the known tms-12 mutant. The tms-12 mutation was cloned and sequenced, revealing a nonsense codon that was predicted to result in an amber fragment that was about 65% of the wild-type size (elimination of 93 C-terminal residues).

Cloning and characterization of Bacillus subtilis homologs of Escherichia coli cell division genes ftsZ and ftsA

The Bacillus subtilis homolog of the Escherichia coli ftsZ gene was isolated by screening a B. subtilis genomic library with anti-E. coli FtsZ antiserum. DNA sequence analysis of a 4-kilobase region revealed three open reading frames. One of these coded for a protein that was about 50% homologous to the E. coli FtsZ protein. The open reading frame just upstream of ftsZ coded for a protein that was 34% homologous to the E. coli FtsA protein. The open reading frames flanking these two B. subtilis genes showed no relationship to those found in E. coli. Expression of the B. subtilis ftsZ and ftsA genes in E. coli was lethal, since neither of these genes could be cloned on plasmid vectors unless promoter sequences were first removed. Cloning the B. subtilis ftsZ gene under the control of the lac promoter resulted in an IPTGs phenotype that could be suppressed by overproduction of E. coli FtsZ. These genes mapped at 135 degrees on the B. subtilis genetic map near previously identified cell division mutations.

Genetic analysis of bacterial cell division

A relatively small number of cell division genes have been identified in Escherichia coli. The available evidence, however, suggests that several of these genes play a crucial role in the process.

Sequence analysis, transcriptional organization, and insertional mutagenesis of the envA gene of Escherichia coli

The Escherichia coli cell permeability-cell separation gene envA and the region immediately downstream were sequenced. The envA gene consisted of 305 codons which encoded a 34-kilodalton polypeptide that lacked a signal sequence and hydrophobic membrane-spanning regions. The envA1 mutation was determined to be a missense mutation in codon 19 resulting in a change in the amino acid sequence from histidine to tyrosine. Located 299 base pairs downstream of the envA gene was an unidentified open reading frame consisting of 147 codons. This open reading frame was followed by an additional open reading frame starting 59 base pairs further downstream and corresponded to the secA gene. A transcription terminator was located just downstream of envA on a fragment that contained a sequence corresponding to a typical rho-independent terminator. Transcription of envA and the upstream fts genes terminated at this terminator and was probably uncoupled from the downstream genes, including secA. Gene disruption experiments indicated that the envA gene was an essential gene.

Analysis of cell division gene ftsZ (sulB) from gram-negative and gram-positive bacteria

The ftsZ (sulB) gene of Escherichia coli codes for a 40,000-dalton protein that carries out a key step in the cell division pathway. The presence of an ftsZ gene protein in other bacterial species was examined by a combination of Southern blot and Western blot analyses. Southern blot analysis of genomic restriction digests revealed that many bacteria, including species from six members of the family Enterobacteriaceae and from Pseudomonas aeruginosa and Agrobacterium tumefaciens, contained sequences which hybridized with an E. coli ftsZ probe. Genomic DNA from more distantly related bacteria, including Bacillus subtilis, Branhamella catarrhalis, Micrococcus luteus, and Staphylococcus aureus, did not hybridize under minimally stringent conditions. Western blot analysis, with anti-E. coli FtsZ antiserum, revealed that all bacterial species examined contained a major immunoreactive band. Several of the Enterobacteriaceae were transformed with a multicopy plasmid encoding the E. coli ftsZ gene. These transformed strains, Shigella sonnei, Salmonella typhimurium, Klebsiella pneumoniae, and Enterobacter aerogenes, were shown to overproduce the FtsZ protein and to produce minicells. Analysis of [35S]methionine-labeled minicells revealed that the plasmid-encoded gene products were the major labeled species. This demonstrated that the E. coli ftsZ gene could function in other bacterial species to induce minicells and that these minicells could be used to analyze plasmid-endoced gene products.

Reversibility of SOS-associated division inhibition in Escherichia coli

In Escherichia coli the SOS response, induced by DNA-damaging treatments, includes two systems of cell division inhibition, SfiA and SfiC, which are thought to prevent cell division by interacting with the division protein FtsZ. It is shown here that SfiA-mediated division inhibition is readily reversible, even in the absence of de novo protein synthesis, suggesting that functional FtsZ molecules can be recovered from SfiA-FtsZ complexes. The action of SfiC, on the other hand, is essentially irreversible; induction by expression of the recA (Tif) mutation for 60 min results in division inhibition that continues for at least 180 min after the end of the induction period. An excess of the presumed target molecule FtsZ, furnished by a multicopy plasmid, suppresses the action of SfiA but not SfiC. Simultaneous induction of SfiA and SfiC results in irreversible division inhibition, showing that SfiC is epistatic to SfiA. The irreversibility of SfiC action is most readily accounted for by assuming that the SfiC product, unlike SfiA, is stable. The reversibility of SfiA action is slower in a lon mutant, in which the SfiA protein is partially stabilized. From the kinetics of division resumption in the absence of protein synthesis, we estimated the in vivo half-life of the SfiA protein to be 10 min in a lon+ strain and 170 min in a lon mutant.

Overproduction of FtsZ suppresses sensitivity of lon mutants to division inhibition

Escherichia coli lon mutants are sensitive to UV light and other DNA-damaging agents. This sensitivity is due to the loss of the lon-encoded ATP-dependent proteolytic activity which results in increased stability of the cell division inhibitor SulA. Introduction of the multicopy plasmid pZAQ containing the ftsZ gene, which is known to increase the level of FtsZ, suppressed the sensitivity of lon mutants to the DNA-damaging agents UV and nitrofurantoin. Alterations of pZAQ which reduced the expression of ftsZ reduced the ability of this plasmid to suppress the UV sensitivity. Examination of the kinetics of cell division revealed that pZAQ did not suppress the transient filamentation seen after exposure to UV, but did suppress the long-term inhibition that is normally observed. lon strains carrying pZAQ could stably maintain a multicopy plasmid carrying sulA (pBS2), which cannot otherwise be introduced into lon mutants. In addition, the increased temperature sensitivity of lexA(Ts) strains containing pBS2 was suppressed by pZAQ. These results suggest that SulA inhibits cell division by inhibiting FtsZ and that this interaction is stoichiometric.

Overproduction of FtsZ induces minicell formation in E. coli

The ftsZ gene in E. coli K-12 is an essential cell division gene. We report that a two to sevenfold increase in the level of the FtsZ protein resulted in induction of the minicell phenotype. An increase in the level of FtsZ beyond this range resulted in an inhibition of all cell division. Unlike the classical minicell mutant, the formation of minicells induced by increased levels of FtsZ did not occur at the expense of normal divisions, indicating that increasing FtsZ resulted in additional division events per cell cycle. In addition, increased FtsZ caused cell division to be initiated earlier in the cell cycle. These results are consistent with the level or activity of FtsZ controlling the frequency of cell division in E. coli.

Structure and expression of the cell division genes ftsQ, ftsA and ftsZ

The essential cell division genes ftsQ, ftsA and ftsZ map in a cluster of cell envelope genes located at two minutes on the Escherichia coli genetic map and appear to constitute an atypical operon. These closely clustered genes are all transcribed in the same direction and yet each of these genes was independently cloned on a multicopy plasmid and found to be expressed. Tn5 insertion mutagenesis of this region confirmed that these genes were independently expressed, indicating that each of these genes has its own promoter. However, maximum expression of the distal ftsZ gene required the promoter for the proximal ftsQ gene. The proximal ftsZ promoter was located within the ftsA structural gene and the proximal ftsA promoter was located within the ftsQ structural gene. No transcription terminators were evident from the DNA sequence analysis of this region. The sequence analysis also revealed that the termination codon for ftsQ overlapped the initiation codon of the ftsA gene and that the ftsA and ftsZ genes were separated by 60 base-pairs. The ftsQ gene product has a calculated Mr of 31,432 and the ftsA gene product has a calculated Mr of 45,327. The structure and expression of these genes are discussed.

The nucleotide sequence of the essential cell-division gene ftsZ of Escherichia coli

The nucleotide sequence of a 1.8-kb fragment of Escherichia coli DNA containing the essential cell division gene ftsZ is reported. The FtsZ protein has an Mr of 40294 and has 23% charged residues with a calculated isoelectric point of 4.9. The codon usage of the ftsZ gene reflects that of a highly expressed gene. Also located on this DNA fragment is the 3' end of the ftsA gene and the 5' end of the envA gene. These designations were confirmed by locating Tn5 insertions within the ends of these genes that inactivate each of these genes. A potential promoter for ftsZ overlapped the 3' end of the ftsA gene. A Tn5 insertion was located within the 3' end of the ftsA and within this potential promoter. No transcription terminators were evident between ftsA and ftsZ or between ftsZ and envA.

Kinetics of phosphate incorporation into adenosine triphosphate and guanosine triphosphate in bacteria

Escherichia coli cells were labeled with (33)PO(4) for several generations and then (32)PO(4) was added, samples were taken at various times, and adenosine triphosphate (ATP) and guanosine triphosphate (GTP) were purified from an acid extract. The ratio of (32)P/(33)P in each of the six phosphate species was determined and compared to the ratio expected at isotopic steady state. The rate constants of the simplest network of pools which could explain the data, and were consistent with metabolic pathways, were then calculated. In the range of times studied (30 to 3,000 s at 21.5 C) the following generalizations can be made. (i) The gamma-phosphate at ATP requires 2,200 s to achieve 50% of its final specific activity and behaves as if it were exchanging with another species 4.4 times as large, with this other species turning over in 540 s. (ii) The beta-phosphate of ATP and the gamma-phosphate of GTP are in equilibrium with the gamma-phosphate of ATP. (iii) The specific activity of the gamma-phosphate of GTP lags 66 s behind the gamma-phosphate of ATP. (iv) The alpha-phosphates of ATP and GTP have equal specific activities which lag far behind the other four phosphates. A single precursor pool with a turnover time of 416s fits the data best, although a fair fit can be obtained assuming several sequential precursor pools with much shorter turnover times. These findings are consistent with known biosynthetic pathways and the probable flows through them, including the relationship of nucleotide biosynthesis to ribonucleic acid synthesis and turnover.