Box 12.2The link between genome replication and sporulation in Bacillus

A combination of genetic and biochemical studies have shown how sporulation in Bacillus subtilis is coordinated with genome replication.

A prerequisite for sporulation in B. subtilis is that the bacterium replicates its genome. It must do this in order to have two complete copies, one to be retained by the mother cell and one to packaged into the spore. How are the two processes - genome replication and sporulation - coordinated?

Genetic studies indicate a central role for sda

Many advances in understanding sporulation in Bacillus have depended on the study of mutants that either cannot sporulate or follow an aberrant sporulation pathway. One such mutant is dnaA1, which is unable to initiate the sporulation process. The function of the DnaA protein, specified by dnaA1, is well known. It is a DNA-binding protein that attaches to the origin of replication of the Bacillus genome and initiates the series of events that lead to genome replication (Section 13.2.1). The dnaA1 mutation has no effect on replication, showing that the resulting change in DnaA does not disrupt this protein's ability to bind to the replication origin, but it does prevent sporulation. Further study of dnaA1 might therefore reveal the link between replication and sporulation.

One of the standard ways of finding out more about a bacterial mutant is to search for additional mutations that reverse the effects of the one being studied. These are called suppressor mutations and they can be generated simply by exposing the mutant bacteria to ultraviolet radiation or to a chemical mutagen (Section 14.1.1), and then examining the cells for ones that have regained the wild-type characteristics. When this experiment was tried with dnaA1 mutants, five such revertants were obtained. Genetic mapping (Section 5.2.4) showed that all five of these revertants carried mutations in a short previously unrecognized gene that the researchers named sda, standing for suppressor of dnaA1. The Sda protein must be the intermediary between genome replication and sporulation.

Biochemical studies of Sda

The next step was to identify the biochemical activity of the Sda protein. Attention was focused on a possible interaction between Sda and one of the proteins involved in the cascade of events that begins with receipt by the bacterium of an appropriate extracellular signal and culminates in phosphorylation of SpoOA, the master switch that sets the sporulation pathway in motion (see Figure 12.19). The initial response to the extracellular signals that trigger sporulation is provided by three kinases - KinA, KinB and KinC - which phosphorylate themselves and then pass the phosphate via SpoOF and SpoOB to SpoOA.

Image ch12fb1.jpg

It was immediately discovered that Sda inhibits the activity of KinA, and possibly also has an effect on KinB. By inhibiting one or both of these kinases, Sda could indirectly prevent SpoOA from becoming phosphorylated, and hence could block initiation of the sporulation pathway.

The link between replication and sporulation

Identification of the sda gene as key player in sporulation, and elucidation of the effects of the Sda protein on the KinA and KinB kinases, has enabled models for the link between genome replication and sporulation to be constructed. These models envisage that a signal passes between DnaA and Sda, indicating that genome replication has been successfully completed and that sporulation can commence. In response to this signal, Sda lifts its inhibition of KinA and KinB, enabling these kinases to respond, in turn, to the extracellular signals that initiate sporulation. The model is tentative and there are a number of unresolved issues, but it is a good working hypothesis on which to base future research.

References

  1. Burkholder WF, Kurtser I, Grossman AD. Replication initiation proteins regulate a developmental checkpoint in Bacillus subtilis. Cell. (2001);104:269–279. [PubMed: 11207367]
  2. Michael WM. Cell cycle: Connecting DNA replication to sporulation in Bacillus. Curr. Biol. (2001);11:R443–R445. [PubMed: 11516668]

From: Chapter 12, Regulation of Genome Activity

Cover of Genomes
Genomes. 2nd edition.
Brown TA.
Oxford: Wiley-Liss; 2002.
Copyright © 2002, Garland Science.

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