A new gene controlling the frequency of cell division per round of DNA replication in Escherichia coli

Conversion to bidirectional replication after unidirectional initiation from R1 plasmid origin integrated at oriC in Escherichia coli

The cell division phenotypes of Escherichia coli with its chromosome replication driven by oriR (from plasmid R1) were examined by fluorescence microscopy and flow cytometry. Chromosome replication patterns in these strains were followed by marker frequency analyses. In one of the strains, the unidirectional oriR was integrated so that the replication fork moved clockwise from the oriC region, and bacterial growth and division were similar to those of the wild-type parent. The bacteria were able to convert the unidirectional initiation from oriR into bidirectional replication. The site for conversion of uni- to bidirectional replication seemed to be localized and could be mapped genetically within 6 min to the immediate right of the minimal oriC. Replication starting in the counterclockwise direction from the R1 replicon integrated at the same site in the opposite orientation could not be described as either bi- or unidirectional, as no single predominant origin could be discerned from the more or less flat marker frequency pattern. These strains also showed extensive filamentation, irregular nucleoid distribution and the presence of anucleate cells, indicative of segregation and division defects. Comparison among intR1 derivatives differing in the position of the integrated oriR relative to the chromosome origin suggested that the oriC sequence itself was dispensable for the conversion to bidirectionality. However, passage of the replication fork over the 6 min region to the right of oriC seemed important for the bidirectional replication pattern and normal cell division phenotype.

Escherichia coli DNA topoisomerase I and suppression of killing by Tn5 transposase overproduction: topoisomerase I modulates Tn5 transposition

Tn5 transposase (Tnp) overproduction is lethal to Escherichia coli. The overproduction causes cell filamentation and abnormal chromosome segregation. Here we present three lines of evidence strongly suggesting that Tnp overproduction killing is due to titration of topoisomerase I. First, a suppressor mutation of transposase overproduction killing, stkD10, is localized in topA (the gene for topoisomerase I). The stkD10 mutant has the following characteristics: first, it has an increased abundance of topoisomerase I protein, the topoisomerase I is defective for the DNA relaxation activity, and DNA gyrase activity is reduced; second, the suppressor phenotype of a second mutation localized in rpoH, stkA14 (H. Yigit and W. S. Reznikoff, J. Bacteriol. 179:1704-1713, 1997), can be explained by an increase in topA expression; and third, overexpression of wild-type topA partially suppresses the killing. Finally, topoisomerase I was found to enhance Tn5 transposition up to 30-fold in vivo.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Examination of the Tn5 transposase overproduction phenotype in Escherichia coli and localization of a suppressor of transposase overproduction killing that is an allele of rpoH

Tn5 transposase (Tnp) overproduction is lethal to Escherichia coli. Tnp overproduction causes cell filamentation, abnormal chromosome segregation, and an increase in anucleated cell formation. There are two simple explanations for the observed phenotype: induction of the SOS response or of the heat shock response. The data presented here show that overproduction of Tnp neither induces an SOS response nor a strong heat shock response. However, our experiments do indicate that induction of some sigma32-programmed function(s) (either due to an rpoH mutation, a deletion of dnaK, or overproduction of sigma32) suppresses Tnp overproduction killing. This effect is not due to overproduction of DnaK, DnaJ, or GroELS. In addition, Tnp but not deltall Tnp (whose overproduction does not kill the host cells) associates with the inner cell membrane, suggesting a possible correlation between cell killing and Tnp membrane association. These observations will be discussed in the context of a model proposing that Tnp overproduction titrates an essential host factor(s) involved in an early cell division step and/or chromosome segregation.

Studies on the alteration of chromosome copy number and cell division potential in a dnaA mutant of Escherichia coli

The dnaA167 mutant of Escherichia coli, N167, maintains, on the average, two replicating chromosomes per cell at the permissive growth temperature of 30 degrees C and only one per cell at the higher permissive growth temperature of 38 degrees C. When the growth temperature of this mutant is changed from 30 degrees to 38 degrees C the cells rapidly readjust their chromosome copy number from two to one. I have examined the kinetics of this transition with reference to DNA replication and cell division. My results indicate that this mutant uncouples cell division from chromosome duplication to achieve the appropriate copy number, suggesting that the dnaA gene product may be involved in the coordination between these two cellular events.

Analysis of ftsZ mutations that confer resistance to the cell division inhibitor SulA (SfiA)

In Escherichia coli, the ftsZ gene is thought to be an essential cell division gene. Several dominant mutations that make lon mutant cells refractory to the cell division inhibitor SulA, sulB9, sulB25, and sfiB114, have been mapped to the ftsZ gene. DNA sequence analysis of these mutations and the sfiB103 mutation confirmed that all of these mutations mapped within the ftsZ gene and revealed that the two sulB mutations were identical and by selection for resistance to higher levels of SulA, contained a second mutation within the ftsZ gene. We therefore propose that these mutations be redesignated ftsZ(Rsa) for resistance to SulA. A procedure involving mutagenesis of ftsZ cloned on low-copy-number vectors was used to isolate three additional ftsZ(Rsa) mutations. DNA sequence analysis of these mutations revealed that they were distinct from the previously isolated mutations. One of these mutations, ftsZ3(Rsa), led to an altered FtsZ protein that could no longer support cell growth but still conferred the Rsa phenotype in the presence of ftsZ+. In addition to being resistant to SulA, all ftsZ(Rsa) mutations also conferred resistance to a LacZ-FtsZ hybrid protein (ZZ). One possibility is that FtsZ functions as a multimer and that FtsZ(Rsa) mutant proteins have an increased ability for multimerization, making them resistant to SulA and ZZ.

Logic of the Escherichia coli cell cycle

The logic of Escherichia coli's responses to environmental changes gives hope that its cell cycle will be equally well designed. During growth in a constant environment, internal signals trigger cell-cycle events such as replication initiation and cell division. Internal signals must also provide the cell with information about its present state, enabling it to coordinate the synthesis of cytoplasm, DNA and cell wall and maintain proper cell shape and composition. How the cell regulates these aspects of its growth is a fascinating--and as yet unfinished--story.

Chromosome replication does not trigger cell division in E. coli

An essential part of the chromosome replication origin of E. coli K-12 and B/r was replaced by the plasmid pOU71. The average initiation mass of replication for pOU71 decreases with increasing temperature. The constructed strains were grown exponentially at different temperatures, and cell sizes and DNA content were measured by flow cytometry. The average DNA content increased with increasing temperature, but the cell size distribution was largely unaffected. Furthermore, cells in which DNA replication had not yet initiated (cells in the B period) became less abundant with increasing temperature. The increased DNA content could not be explained by an increase in the length of the C period. It is concluded that chromosome replication does not trigger cell division in E. coli, but that the chromosome replication and cell division cycles of E. coli run in parallel independently of each other.

A cell division regulatory mechanism controls the flagellar regulon in Escherichia coli

The formation of flagella in various thermosensitive (Ts) cell division mutants of Escherichia coli was examined at the nonpermissive temperature. The number of flagella per unit cell length decreased sharply after shifting the culture temperature from 30 degrees to 40 degrees C in the following Ts mutants: ftsC108, ftsD1033, ftsE1181, ftsF1141, ftsG29, ftsZ84, parA110, dnaB42, nrdB, and dnaG. It was found that transcription of genes responsible for the formation and/or function of flagella (hag, fla, mot, che) decreased significantly at 40 degrees C. However, in the ftsI730 mutant at the nonpermissive temperature, or in penicillin G treated wild-type cells, cell division was blocked but formation of flagella continued. Moreover, when the cfcA1 mutation, of a gene involved in coordinating DNA replication and cell division, was introduced into the dnaB42 mutant strain, inhibition of cell division and also of formation of flagella at 40 degrees C was relaxed. These results indicate that the flagellar regulon is under the control of a cell division regulatory mechanism.