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10 Two-Component Signal Transduction Systems of the Myxobacteria
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Myxobacteria live in an ever-changing environment and therefore require mechanisms to couple perception of environmental change with appropriate behavioral responses. The evolutionary success of two-component system (TCS) signaling pathways apparently stems from their adaptability to the regulation of diverse physiological processes, and this feature is illustrated well by the known TCSs of myxobacteria. The recent availability of genome sequences of four myxobacteria has enabled a comparative genomic analysis of TCS genes in myxobacterial genomes. TCSs can be consistently grouped into particular subfamilies by applying several different assessment criteria, including gene organization, domain architecture, and phylogenetic relationships. The major families of TCSs are usually named after archetypal family members from Escherichia coli. Phosphoaspartate residues in response regulators can also be hydrolyzed by extrinsic phosphatases. Surprisingly, there are no homologues of the Rap, Spo0E, YisI, YnzD, or CheZ phosphatases encoded in the Myxococcus xanthus genome, suggesting that modulation of signal flow by regulated phosphoaspartate phosphatase activity is not generally adopted by the myxobacteria. With the sequencing of multiple myxobacterial genomes it has become possible to use comparative genomics to gain novel insights into the TCSs of myxobacteria. Genomes can be assessed for the presence or absence of specific TCS homologues, lineage-specific changes in TCS properties can be identified, and in some cases changes in gene organization can guide searches for partner proteins.
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Typical TCSs and phosphorelays. Domain architecture graphics obtained using the SMART tool (http://smart.embl-heidelberg.de). Phosphotransfer reactions are indicted with arrows. (A) The PhoBR and CheAY TCSs of E. coli. PhoR contains an orthodox transmitter domain comprising a HisKA phosphotransfer domain and a HATPase domain and is activated in response to low extracellular phosphate concentrations. This leads to phosphorylation of the DNA-binding response regulator PhoB, which activates transcription of phosphate-scavenging genes. CheA possesses an unorthodox transmitter domain, which contains a phospho-accepting Hpt domain and HATPase domain, with a vestigial form of the HisKA domain (lacking a phospho-accepting histidine residue) retained as a dimerization domain. Sensing of attractants/repellants by the Tar MCP activates CheA, which phosphor-ylates CheB and CheY. (B) The phosphorelay regulating initiation of sporulation in B. subtilis. Spo0F is phosphorylated by at least two kinases, including KinA and KinB. Phosphoryl groups are passed from Spo0F to Spo0A via a phosphotransfer protein, Spo0B, which is phosphorylated on a histidine residue and structurally resembles both Hpt domains and dimeric H-box/HisKA domains. Spo0A can also be phosphorylated directly by a third histidine kinase, KinC, and when phosphorylated promotes sporulation through changes in gene expression. For details see Stock et al. (1989) and Fabret et al. (1999).
Numbers of TCS genes found in different genomes as a function of genome size. Trend lines are shown for all bacteria (gray) and for four myxobacteria (black). The myxo-bacteria all possess an exceptionally large complement of TCS genes, given their genome size. Nostoc (PCC7120) and Geobacter sulfurreducens (PCA) also have relatively large numbers of TCS genes.
Domain architectures of response regulator families. Graphics of domain organization were obtained using the SMART tool (http://smart.embl-heidelberg.de). Families are named after archetypal members, and examples from M. xanthus are indicated.
Complex TCS gene clusters of M. xanthus. Genes are represented by arrows pointing in the direction of transcription. Domains encoded within each gene are shown as SMART graphics (http://smart.embl-heidelberg.de), with conserved histidine and aspartate residues predicted to be involved in phosphotransfer indicated below each gene.
A map of the M. xanthus genome showing the location of TCS genes. The origin of the genome is defined as the start of the dnaA gene (position 1). Inner and outer rings represent genes coded on the + and — strands, respectively. Figure produced using GenomeDiagram ( Pritchard et al., 2006 ).
Transmembrane histidine kinases (including hybrid kinases) found in different genomes as a function of total numbers of histidine kinases (including hybrids). Trend lines are shown for all bacteria (gray) and for four myxobacteria (black). Ad, Anaeromyxobacter dehalogenans; Mx, Myxococcus xanthus; Sa, Stigmatella aurantiaca; Sc, Sorangium cellulosum. Most myxobacteria (with the exception of A. dehalogenans) have exceptionally low proportions of TM histidine kinases, implying an unusual degree of sensing of intracellular conditions. Two genomes of Nostoc sp. also exhibit evidence of significant intracellular sensing (see Galperin, 2005).
TCSs of M. xanthus a
Organization of TCS genes of M. xanthus and other bacteria a