Sigma Factor Exchange during Bacillus Subtilis Sporulation

University Of Texas Health Science Center San Antonio, San Antonio TX

Investigators

Abstract

Bacillus endospore formation represents a simple differentiation in which a series of physiological and morphological changes leads to a dormant cell form. At an early stage in sporulation, Bacillus subtilis partitions itself into two unequal compartments that are destined for unique fates. The smaller forespore compartment develops into the spore. The larger mother cell compartment engulfs and nurtures the forespore, then lyses to release the mature spore into the environment. Each compartment's program of gene expression is controlled by the sequential appearance of unique transcription factors. Sigma E is the first of the mother cell transcription factors. It is synthesized in the predivisional cell as an inactive proprotein (pro- E ), which later becomes active during a 2 hour period in the mother cell compartment. Maturation of pro- E occurs when a sporulation specific protease (SpoIIGA), activated by a forespore generated signal, cleaves 27 amino acids from the proprotein's amino terminus. The activity of sigma E is restricted by both the time of its processing and its turnover. Mature sigma E is unstable in the mother cell and even the normally stable precursor is subject to compartment-specific degradation in the forespore. The activation of sigma E and its turnover will be examined, using mutational analyses to identify features of sigma E that make it uniquely sensitive to degradation in the mother cell, the factors that lead to its proteolysis in the forespore, and the consequences of preventing sigma E decay on spore gene expression. In other experiments, the nature of the sigma E pro-sequence's block on sigma E -activity will be examined in vitro. Additional assays will test the ability of mature sigma E to compete for RNA polymerase with the sigma factor A that it displaces during sporulation to ask whether this holoenzyme changeover may be due to an inherent property of the sigma factors themselves. Finally, mutational analyses and fluorescent protein fusions will be employed to examine the activation of the SpoIIGA protease and the basis for its localization in the mother cell. Regulation of cell fate by the selective activation and degradation of transcription factors is likely to be widespread in developmental processes. Bacillus sporulation provides a tractable model system in which such mechanisms can be explored in detail. This project will serve to train graduate and undergraduate students; many of the students are from underrepresented backgrounds.