|Kinscherf, Thomas - UNIV. OF WISCONSIN|
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: July 1, 2003
Publication Date: February 4, 2004
Citation: Willis, D.K., Kinscherf, T.G. 2004. Global regulation in pseudomonas syringae.p. 223-238. In J. L. Ramos (ed.), The Pseudomonads Vol. II. Virulence, gene regulation and metabolism. Kluwer, New York, NY. Technical Abstract: Two questions come rather quickly to mind when one is confronted with writing a chapter on global regulation in Pseudomonas syringae. The first of these is very basic, and goes to the eventual scope of the article: what is Pseudomonas syringae? This is not a straightforward question, as the species is something of a taxonomic mess. P. syringae has long been recognized as a member of the original 'inner circle' of pseudomonads, eventually codified as the rRNA Group I (43). This group was further refined by both general and molecular characteristics into two additional phylogenetic groupings recently described as the intrageneric clusters (IGC) I and II, with P. syringae being in IGC II (58). Detailed analysis of the nucleotide sequences of the gyrB and rpoD genes indicated that the IGC II cluster could be reduced still further to three complexes, with one being the 'P. syringae complex' containing the pathovars and nomenspecies traditionally associated with P. syringae (58). After this point, the taxonomic picture becomes less clear. Plant pathogenic pseudomonads are not readily differentiated by standard phenotypic characteristics and have often been classified largely by host range as pathovars or subspecies of P. syringae (49). This is an unusually heterologous grouping, with DNA hybridization studies, ribotyping, and general characteristics having defined at least six (43), and possibly as many as nine (15), discrete genomospecies within the complex. Some of these have already been proposed to be elevated to species status (15), with more sure to follow. However, as there is no clear agreement as to what should go where, or when it should happen, this situation is not likely to be completely resolved any time soon. It is thus something of an open question as to what data should be included in this review. To simplify matters for this chapter, we have essentially defined P. syringae in the less strict sense of the complex described above, allowing the inclusion of information from a larger group of plant pathogenic pseudomonads. The second question posed is more obvious, and once again goes to the scope of the article: what is global regulation? As with many conceptual terms in molecular biology, a strict definition for global regulation is made difficult by the constant influx of new data that stretches our previous understanding of the ability of bacterial cells to organize and delegate resources. The basic concept of global regulation can be fairly intuitive; one that relatively distinguishes between 'local' regulators that effect the expression of a specific gene group often physically linked to the regulatory gene, and more broadly based regulators that coordinate the expression of multiple gene groups in response to cellular conditions. Thus lacI, the negative regulatory element of the lactose utilization operon, is not a global regulator, since the effects of its regulatory activity are largely confined to the lac operon. The gene rpoH, on the other hand, encodes a sigma factor required for the efficient transcription of a variety of unlinked genes that a bacterial cell needs expressed at elevated temperatures, and thus has global effects within the cellular milieu. To some degree, responsiveness to the environmental condition is usually implicit in discussions of global regulation. We rarely hear the rpoD (sigma70) gene product being described as a global regulator, although it clearly is one in that it controls the expression of a disparate lot of unlinked genes; this distinction appears to reflect the understanding that RpoD, as the major sigma factor, is simply part of the regular cell machinery under 'normal' growth conditions. It is the cell's ability to coordinate specific gene expression in response to the peculiarities of its current situation that defines the global regulatory webs. In this review, we will describe data related to global regulatory elements that have been described in Pseudomonas syringae. It is worth noting that in some cases global effects for a particular element have not been directly demonstrated in P. syringae, but were extrapolated from data in other systems such as P. aeruginosa. Also, while we have endeavored to make this chapter readable by itself, mechanistic details for some of the systems that will be covered extensively elsewhere in the larger volume have been minimized to avoid overlap.