Rebecca LeBard

Quorum Sensing in Extreme Environments

Microbial communication, particularly that of quorum sensing, plays an important role in regulating gene expression in a range of organisms. Although this phenomenon has been well studied in relation to, for example, virulence gene regulation, the focus of this article is to review our understanding of the role of microbial communication in extreme environments. Cell signaling regulates many important microbial processes and may play a pivotal role in driving microbial functional diversity and ultimately ecosystem function in extreme environments. Several recent studies have characterized cell signaling in modern analogs to early Earth communities (microbial mats), and characterization of cell signaling systems in these communities may provide unique insights in understanding the microbial interactions involved in function and survival in extreme environments. Cell signaling is a fundamental process that may have co-evolved with communities and environmental conditions on the early Earth. Without cell signaling, evolutionary pressures may have even resulted in the extinction rather than evolution of certain microbial groups. One of the biggest challenges in extremophile biology is understanding how and why some microbial functional groups are located where logically they would not be expected to survive, and tightly regulated communication may be key. Finally, quorum sensing has been recently identified for the first time in archaea, and thus communication at multiple levels (potentially even inter-domain) may be fundamental in extreme environments.

Analysis of the pSK1 replicon, a prototype from the staphylococcal multiresistance plasmid family

Multidrug-resistant staphylococci often harbour plasmids that carry genes conferring resistance to several antimicrobial compounds. Many of these multiresistance plasmids appear to utilize a related theta-type replication system for which multiresistance plasmid pSK1 serves as a prototype. Essential pSK1 replication elements were identified by cloning segments of the replication region and testing the resulting plasmids for replication proficiency. An iterated region within rep and a DNA segment of up to 68 bp upstream of the rep promoter were both found to be essential for origin activity. The Rep protein was overexpressed as a 6xHis-tagged C-terminal fusion protein and was shown to bind in vitro to four Rep boxes located within the rep coding region. Inactivation of a divergently oriented promoter upstream of rep, designated P(rnaI), resulted in an elevated plasmid copy number. Comparative analyses suggest that the replication systems of many staphylococcal multiresistance plasmids share a similar genetic organization and utilize an antisense-RNA-mediated regulatory mechanism for copy number control.

A multimer resolution system contributes to segregational stability of the prototypical staphylococcal conjugative multiresistance plasmid pSK41

The 46-kb plasmid pSK41 is the prototype of a family of staphylococcal conjugative multiresistance plasmids. Sequence analyses have revealed the presence of a putative resolvase gene, res, on pSK41, and identical or related genes carried by other staphylococcal multiresistance plasmids. Carriage of the res region was found to ameliorate the accumulation of multimeric plasmid forms, and recombinant plasmids encoding a wild-type res gene exhibited greater plasmid segregational stability than counterparts carrying a nonfunctional mutant, irrespective of whether the cognate or a heterologous replication system and host was utilized. In vitro DNA-binding studies demonstrated that purified Res protein binds within the intergenic region upstream of the res coding sequence. Six copies of an imperfect 11-bp repeat sequence were identified within DNA sequences protected by Res in DNAseI footprinting studies, in an arrangement that suggests a typical resolution site organization consisting of three subsites.