Synchronous division induced in Escherichia coli K12 by phage Mu: analysis of DNA topology and gene expression during the cell cycle

The GemA protein of phage Mu and the GyrB gyrase subunit of Escherichia coli: the search for targets and interactions leading to the reversion of Mu-induced mutations

The mutant bacteriophage Mugem2(Ts), known to synchronize the division of infected cells, to relax DNA supercoiling and, as prophage, to give rise to precisely excised revertants, has been thought to overexpress the gemA-mor operon, and genetic evidence suggests that the B subunit of DNA gyrase (GyrB) is the target of action of GemA. In two different double hybrid tests presented here, we find no evidence of GemA-GyrB protein-protein interaction. We do observe a GemA-GemA interaction, however, indicating that GemA can dimerize. In lacZ::Mu lysogens, overexpression of the gemA-mor operon from a plasmid, under control of the L-arabinose inducible p(araBAD) promoter, does not permit the recovery of Lac(+) revertants. These observations suggest that GyrB is not the direct target of GemA action and that the various phenotypes of Mugem2(Ts) are not caused by overexpression of the gemA-mor operon.

Regulation of the bacteriophage Mu gem operon

The gem operon of bacteriophage Mu, responsible for the complex phenomenon of phage conversion, is included in the so called "semiessential early" region of phage DNA. Unlike the other early genes of the phage which are transcribed from the pe promoter, expression of the gem operon is driven by its own promoter, which escapes the control of the repressor. In fact, the transcript corresponding to gem was detected in immune lysogens by using a combined reverse transcription and a subsequent amplification of the resulting cDNA. The transcription initiation site from pgem was determined by primer extension mapping experiments and localized at 8217 bp from the left end of phage DNA. Two elements which could perform the negative control of gem were also identified. The first is a phage product, GemB, which presumably interferes with gem expression at a posttranscriptional level, whereas the second is a structural element, an inverted repeat immediately downstream of pgem, which acts as a terminator for the transcripts starting from pe. These transcripts could regulate gem expression by interfering with the initiation of transcription from pgem.

Mu gem2ts DNA integration is not necessary for induction of synchrony of cell division in Escherichia coli K12

The gem2ts mutant of bacteriophage Mu induced synchrony of cell division on bacteria surviving infection. Induction of synchronous growth could also be observed as a response to the entire infected bacterial population, as in the case of infection of hic mutants, a peculiar class of gyrB alleles. After Mu wild-type or Mu gem2ts infection of hic mutants, there was a lack of viral DNA integration and replication, while phage gene expression (including that of A gene, coding for the transposase) seemed to be quite normal. These data indicate that the mechanism of bacterial synchronization induced by Mu gem2ts does not require integration nor replication of the phage DNA.

Dependency of Escherichia coli cell-division size, and independency of nucleoid segregation on the mode and level of ftsZ expression

Expression of ftsZ in strain VIP205 is dissociated from its natural promoters, and is under the control of an inducible tac promoter. This abolishes the oscillation in ftsZ transcription observed in the wild type, allowing different levels of ftsZ expression. We demonstrate that this construction does not affect the expression of other genes, and has no effects on replication or nucleoid segregation. A shift in IPTG from 30 microM, that supports division at wild-type sizes, to lower (6 microM) or higher (100 microM) concentrations, indicates that VIP205 cells can divide within a broad range of FtsZ concentrations. Analysis of the morphological parameters during the transition from one IPTG concentration to another suggests that the correct timing of ftsZ expression, and the correct FtsZ concentration, are required for division to occur at normal cell sizes. After a transient division delay during the transition to lower IPTG concentrations, cells in which ftsZ is expressed continuously (yielding 80% of the wild-type FtsZ levels) divide with the same division time as the wild type, but at the expense of becoming 1.5 times larger. A precise control of ftsZ expression is required for normal division, but the existence of additional regulators to maintain the correct timing during the cell cycle cannot be ruled out.

Reversal of Mu gem2ts-induced mutations

Mutations induced by the integration of a Mu gem2ts mutant prophage can revert at frequencies around 1 x 10(-6), more than 10(4)-fold higher than that obtained with Mu wild-type. Several aspects characterize Mu gem2ts precise excision: (i) the phage transposase is not involved; (ii) the RecA protein is not necessary; and (iii) revertants remain lysogenic with the prophage inserted elsewhere in the host genome. In addition, prophage re-integration seems to be non-randomly distributed, whereas Mu insertion into the host genome is a transposition event without any sequence specificity. In this paper, we describe that the site of re-integration somehow depends on the original site of insertion. Two alternative models are proposed to explain the strong correlation between donor and receptor sites.

Overview of controls in the Escherichia coli cell cycle

The harmonious growth and cell-to-cell uniformity of steady-state bacterial populations indicate the existence of a well-regulated cell cycle, responding to a set of internal signals. In Escherichia coli, the key events of this cycle are the initiation of DNA replication, nucleoid segregation and the initiation of cell division. The replication initiator is the DnaA protein. In nucleoid segregation, the MukB protein, required for proper partitioning, may be a member of the myosin-kinesin superfamily of mechanoenzymes. In cell division, the FtsZ protein has a tubulin motif, is a GTPase and polymerizes in a ring around midcell during septation; the FtsA protein has an actin-like structure. The nature of the internal signals triggering these events is not known but candidates include cell mass, the superhelical density of the chromosome and the concentration of two regulatory nucleotides, cyclic AMP and ppGpp. The involvement of cytoskeletal-like proteins in key cycle events encourages the notion of a fundamental biological unity in cell cycle regulation in all organisms.

Relationship between ftsZ gene expression and chromosome replication in Escherichia coli

Transcriptional levels within the ftsQAZ region of the Escherichia coli chromosome were correlated with chromosome replication and the division cycle. The transcripts were measured either in synchronous cultures generated by the baby machine technique or in dnaC2(Ts) mutants that had been aligned for initiation of chromosome replication by temperature shifts. Transcription within the ftsZ reading frame was found to fluctuate during the cell cycle, with maximal levels about midcycle and a minimum level at division, in cells growing with a doubling time of 24 min at 37 degrees C. Examination of transcription in dnaC(Ts) mutants aligned for chromosome replication indicated that the periodicity was due to a reduction in transcripts coincident with replication of the ftsQAZ region. Transcription originating upstream of the ftsA gene exhibited the periodicity and accounted for a significant proportion of the transcripts entering ftsZ. The most obvious interpretation of the data is that replication of the region transiently inhibits transcription, but alternative explanations have not been ruled out. However, no other relationship between transcription and either replication or division was detected.

Expression of the division-controlling gene ftsZ during growth and sporulation of the filamentous bacterium Streptomyces griseus

The branched, filamentous cells of Streptomyces form two different types of septum: those found infrequently in vegetative mycelia and those that form the boundaries of developing spores. To begin to understand the role of cell septation events in the Streptomyces life cycle, we have isolated the ftsZ locus from Streptomyces griseus, an organism that undergoes sporulation on solid surfaces and in liquid culture. The nucleotide sequence of the cloned DNA indicates that ftsZ in S. griseus lies within a region containing other genes likely to be involved in cell division and cell wall biogenesis. A gene (ORF1) showing significant similarity to ftsQ maps a short distance upstream from ftsZ, but there is no evidence for an ftsA homologue between ftsZ and ORF1. Transcription analysis suggests that ftsZ is expressed during both vegetative growth and sporulation. Immunoblots of soluble protein preparations from vegetative and sporulating mycelia indicate that FtsZ is present at similar levels during growth and differentiation. There appears to be only one ftsZ gene in S. griseus. We interpret these results to indicate that any temporal regulation of FtsZ that may be necessary for the enhanced synthesis of septa during sporulation of S. griseus is likely to occur predominantly at the level of activity rather than synthesis.

A novel illegitimate recombination event: precise excision and reintegration with the Mu gem mutant prophage

The bacteriophage Mu is known to insert its DNA more or less randomly within the Escherichia coli chromosome, as do transposable elements, but unlike the latter, precise excision of the prophage, thereby restoring the original sequence, is not observed with wild-type Mu, although it has been reported with certain defective mutants. We show here that the mutant prophage Mu gem2ts can excise precisely from at least three separate loci -- malT, lac and thyA (selected as Mal+, Lac+ and Thy+, respectively). This excision occurs under permissive conditions for phage development, is observed in fully immune (c+) lysogens, and is independent of RecA and of Mu transposase. Mu gemts2 excision is invariably accompanied by reintegration of a Mu gem2ts prophage elsewhere in the chromosome. In the case of Mal+ revertants, this prophage is systematically located at 94 min on the E. coli chromosome. Mu gem2ts excision therefore sheds some light on the long-standing paradox of the lack of precise Mu excision.

The Mu gem operon: its role in gene expression, recombination and cell cycle

Two genes, gemA and gemB, belong to the gem operon located in the semi-essential early region of bacteriophage Mu. The product of gemA modulates the expression of various host genes, including cell division and DNA replication genes. In addition, GemA is also responsible for decreasing host DNA gyrase activity and for DNA relaxation. The product of gemB is involved in Mu late gene transcriptional transactivation. Phage mutants such as Mu gem2ts have strong effects on the bacterial host: i) infected bacteria become unable to grow in minimal synthetic medium and behave phenotypically as relA- mutants; ii) survivors of the infection are re-programmed in their cell cycles, with synchronous cell divisions, cyclical waves of DNA relaxation and recoiling and; iii) Mu gem2ts prophages excise precisely their DNA from the initial integration site and re-integrate in other non-randomly distributed sites. Neither the phage transposase nor the host RecA protein are implicated in this process.

Expression of the Escherichia coli ftsZ gene: trials and tribulations of gene fusion studies

The ftsZ gene of Escherichia coli, which codes for an essential cell division protein, is subjected to multiple regulation, as shown in part with studies using an ftsZ::lacZ operon fusion located on phage lambda JFL100. Using this same fusion, we sought to isolate regulatory mutants overexpressing ftsZ by selecting mutants able to grow on lactose. One Lac+ mutant was obtained which overexpressed the ftsZ::lacZ fusion 70-fold. The mutation responsible for the overexpression lies in a new gene, cot, located near 56 min on the E. coli genetic map. The cot mutation probably affects the transcription of a chromosomal open reading frame, ORF1, lying downstream of the bioA gene and adjacent to the ftzZ::lacZ fusion of the lambda JFL100 prophage integrated at att lambda. Using an ftsZ84(Ts) strain, in which there was a double selection for overexpression of both ftsZ::lacZ and ftsZ+, no Lac+Tr mutants were obtained from 3.6 x 10(10) bacteria; the introduction of a mutL allele, increasing spontaneous base substitution mutation rates 75-fold, did not permit us to isolate such a mutant. We conclude that Lac+ ftsZ-constitutive mutations cannot be obtained in lambda JFL100 lysogens by a single base substitution.

An amino-proximal domain required for the localization of FtsQ in the cytoplasmic membrane, and for its biological function in Escherichia coli

The location of FtsQ, an Escherichia coli protein essential for cell division, is, under physiological conditions, in the cytoplasmic membrane facing towards the periplasmic space. An amino-proximal hydrophobic domain is required for FtsQ to reach its location and for its activity in the cell. Overexpression of modified forms of FtsQ is deleterious for the cell.

The role of the 'gearbox' in the transcription of essential genes

Regulation of transcription occurs at different levels, one being in the presence of sequences specifically recognized by different forms of RNA polymerase, i.e. the promoters. Three different kinds of promoter are defined according, among other things, to their dependence on the growth rate of the cell: the 'house-keeper' promoter of many metabolic genes, the stringent promoter found at several rRNA and ribosomal protein genes, and the 'gearbox' at genes whose products are required at higher relative amounts at lower growth rates. The identified gearbox promoters of Escherichia coli share specific homologies in the -10, -35 and upstream regions. Although there may be different types of gearbox promoters, the -10 sequence of one of these promoters has been found to be essential for functioning as a gearbox. This suggests the existence of specific sigma factors for its transcription. RpoS (KatF) is a likely candidate for being one of these sigma factors. Computer simulation allows us to predict that such sigma factors should, in turn, be expressed following a gearbox mode, which would then imply the existence of self-regulated loops contributing to the expression of some genes of bacterial division.