Induction of prophage SPO2 in Bacillus subtilis: prophage excision in the absence of bacterial or bacteriophage DNA synthesis

The Min system and nucleoid occlusion are not required for identifying the division site in Bacillus subtilis but ensure its efficient utilization

Precise temporal and spatial control of cell division is essential for progeny survival. The current general view is that precise positioning of the division site at midcell in rod-shaped bacteria is a result of the combined action of the Min system and nucleoid (chromosome) occlusion. Both systems prevent assembly of the cytokinetic Z ring at inappropriate places in the cell, restricting Z rings to the correct site at midcell. Here we show that in the bacterium Bacillus subtilis Z rings are positioned precisely at midcell in the complete absence of both these systems, revealing the existence of a mechanism independent of Min and nucleoid occlusion that identifies midcell in this organism. We further show that Z ring assembly at midcell is delayed in the absence of Min and Noc proteins, while at the same time FtsZ accumulates at other potential division sites. This suggests that a major role for Min and Noc is to ensure efficient utilization of the midcell division site by preventing Z ring assembly at potential division sites, including the cell poles. Our data lead us to propose a model in which spatial regulation of division in B. subtilis involves identification of the division site at midcell that requires Min and nucleoid occlusion to ensure efficient Z ring assembly there and only there, at the right time in the cell cycle.

How organisms regulate biological processes so that they occur at the correct place within the cell is a fundamental question in research. Spatial regulation of cell division is vital to ensure equal partitioning of DNA into newborn cells. Correct positioning of the division site at the cell centre in rod-shaped bacteria is generally believed to occur via the combined action of two factors: (i) nucleoid (chromosome) occlusion and (ii) a set of proteins known collectively as the Min system. The earliest stage in bacterial cell division is the assembly of a ring, called the Z ring, at the division site. Nucleoid occlusion and Min work by preventing Z ring assembly at all sites along the cell other than the cell centre. Here we make the surprising discovery that, in the absence of both these factors, Z rings are positioned correctly at the division site, but there is a delay in this process and it is less efficient. We propose that a separate mechanism identifies the division site at midcell in rod-shaped bacteria, and nucleoid occlusion and Min ensure that the Z ring forms there and only there, at the right time and every time.

High-frequency elimination of SP02 prophage from Bacillus subtilis by plasmid transformation

Transformation of competent Bacillus subtilis lysogenic for SP02 with any of three plasmids (pCM194, pUB110, pAM77) generates drug-resistant transformants of which 5 to 20% have lost the infectivity and immunity associated with the SP02 prophage. Such cured derivatives can be again lysogenized with SP02 and again cured by introduction of a different plasmid. Elimination of the SP02 prophage was not detected when plasmids were introduced by PBS1 transduction or by transformation of protoplasts. Similarly, transformants of B. subtilis selected for chromosome markers retained the prophage. The phi 105 prophage was not eliminated from competent B. subtilis transformed with plasmids.

Induction of prophage SPO2 in Bacillus subtilis: isolation of excised prophage DNA as a covently closed circle

Bacillus subtilis tryC2, thyA, thyB, lysogenic for the phage DNA polymerase negative mutant SPO2 susL244, was induced under conditions preventing phage and bacterial DNA synthesis. The biological activity of DNA from induced cells and from uninduced controls was assayed by transformation and transfection, respectively. About 50% of the phage DNA biological activity in DNA extracted from induced cells was resistant to exposure to pH 11.8 TO 11.9. This DNA was operationally defined as alkali-resistant phage DNA. Transforming bacterial DNA from uninduced or induced cells and transfecting DNA from uninduced cells were more than 95% inactivated after exposure to high pH. The alkali-resistant phage DNA was characterized by sucrose gradient centrifugation, by centrifugation in cesium chloride-propidium iodide, and by electron microscopy. It was found to consist of a majority of covalently closed circular DNA molecules. Length measurements of a few relaxed circular molecules indicate a molecular weight of these similar to that previously found for mature SPO2DNA. Attempts to isolate similar covalently closed circular phage DNA from induced bacteria lysogenic for SPO2 phage with a functional DNA polymerase gene were unsuccessful. The gene order in mature and prophage SPO2 was determined by rescue of single and double markers from the respective type of DNA. The data obtained show that prophage DNA is (genetically) permuted relative to mature DNA. The phage attachment site is suggested to be located between genes I and J.

Bacteriophages of Bacillus subtilis

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Induction of prophage SPO2 in Bacillus subtilis by 6-(para)-hydroxyphenylazouracil

DNA replication in Bacillus subtilis is reversibly inhibited by 6-(para)-hydroxyphenylazouracil (HpUra) (1971), whereas replication of temperature phage SPO2 is not affected by the drug. Experiments are presented which show that HpUra will induce bacteria lysogenic for SPO2. Also, prophage phi105, which is sensitive to the drug, is induced by HpUra. Induction of SPO2 lysogenics in the presence of HpUra gives selective synthesis of SPO2 DNA.