Genetics: Molecular Biology of genetic recombination; Transformation mechanisms in Streptococus quorum sensing

Homologous recombination is a ubiquitous genetic process that generates diversity without causing genetic chaos. The outline of the mechanisms involved is well known, but many details are still obscure, partly because in vivo the phenomenon is both complex and rare. In the process of genetic transformation in naturally competent (transformable) bacteria, recombination is both relatively simple and frequent. The DNA exchange reaction is reduced to its essence: a single DNA chain from a donor molecule replaces an identical nucleic acid chain segment within the cell chromosome. In addition, the reaction occurs in massive quantities at high efficiencies: >2% of a cell's genes can be replaced within a few minutes. In the widespread human pathogen, Streptococcus pneumoniae, genetic transformation is important for rapid evolution and horizontal gene transfer but also provides powerful genetic tool for molecular analysis of its biology.

In examining the process of genetic transformation in S. pneumoniae, we have found that its high efficiency reflects a brief, coordinated differentiation of virtually all cells in a culture for synthesis of several score of new proteins, just on those rare occasions when a culture becomes transformable. The most prominent of these proteins coats DNA strands upon entry. Another is an extracellular peptide phermone that coordinates the differentiation throughout a culture. Others' roles are not yet fully known. We have recently used DNA microarrays for mRNA expression profiling to identify ~180 genes with strongly altered expression in competent cultures. A few (~8) of these specify a quorum-sensing circuit that senses population density, more specify the machinery for DNA uptake and recombination, and even more have other, yet unknown, functions.

We have created a large mutant library, which will make it possible to study the role of all the unknown competence induced genes mentioned above. Recently, we identified a new competence regulator, ComW, and are characterizing its biochemical role. We are also exploring the regulatory mechanisms that combine to accomplish quorum sensing and link it to global gene expression patterns, and seeking new activities in the DNA recombination pathway.

Representative Publications

Mashburn-Warren L, Morrison DA and Federle MJ (2010) A novel double-tryptophan peptide pheromone controls competence in Streptococcus spp. via an Rgg regulator. Mol. Microbiol. 78(3): 589-606.

Piotrowski A, Luo P and Morrison DA (2009) Competence for genetic transformation in Streptococcus pneumoniae: termination of activity of the alternative sigma factor ComX is independent of proteolysis of ComX and ComW. J. Bacteriol. 191(10): 3359-3366.

Ahlawat S and Morrison DA (2009) ClpXP degrades SsrA-tagged proteins in Streptococcus pneumoniae. J. Bacteriol. 191: 2894-2898.

Oggioni MR and Morrison DA (2008) Cooperative regulation of competence development in Streptococcus pneumoniae: cell-to-cell signaling via a peptide pheromone and an alternative sigma factor. In Chemical Communication among Bacteria (SC Winans and B Bassler, eds), ASM Press, Washington DC, pp. 345-362.

Sung CK and Morrison DA (2005) Two distinct functions of ComW in stabilization and activation of the alternative sigma factor ComX in Streptococcus pneumoniae. J. Bacteriol. 187: 3052-3061.

Peterson SN et al (2004) Identification of competence pheromone responsive genes in Streptococcus pneumoniae by use of DNA microarrays. Molec. Microbiol. 51: 1051-1070.

Luo P and Morrison DA (2003) Transient association of an alternative sigma factor, ComX, with RNA polymer-ase during competence for genetic transformation in Streptococcus pneumoniae. J. Bacteriol. 185: 349-358.