Evolution of Two-Component Signal Transduction Systems

Abstract

To exist in a wide range of environmental niches, bacteria must sense and respond to a myriad of external signals. A primary means by which this occurs is through two-component signal transduction pathways, typically comprised of a histidine kinase that receives the input stimuli and a response regulator that effects an appropriate change in cellular physiology. Histidine kinases and response regulators have an intrinsic modularity that separates signal input, phosphotransfer, and output response; this modularity has allowed bacteria to dramatically expand and diversify their signaling capabilities. Recent work has begun to reveal the molecular basis by which two-component proteins evolve. How and why do orthologous signaling proteins diverge? How do cells gain new pathways and recognize new signals? What changes are needed to insulate a new pathway from existing pathways? What constraints are there on gene duplication and lateral gene transfer? Here, we review progress made in answering these questions, highlighting how the integration of genome sequence data with experimental studies is providing major new insights.

Two-component signal transduction systems in oral bacteria

“…Bacterial species can express several different types of TCS, which differ in structure depending on the types of signals sensed (normally chemical ligands) and in the set of target genes affected. The numbers of TCSs expressed by a species appears to correlate with the size of its genomes and more interestingly with the extent of diversity of the natural niches [21,22]. Table 1 shows the numbers of TCSs expressed for some to the most common bacterial species present in supra-gingival and sub-gingival biofilms.…”

“…Mycoplasma spp. [22], oral species that are mostly restricted to intracellular compartments of the host and/or to anoxic conditions, e.g. Actinobacillus actinomycetencomitans and P. gingivalis , have a limited number (two to seven) of complete TCSs (Table 1).…”

“…Evolutionary analyses of TCSs indicate that bacterial lineages might acquire novel TCSs by gene duplication and lateral gene transfer under specific selective pressures. These ‘novel’ TCSs evolve to assume specific functions, likely to avoid unsuitable cross-talk with other pathways [22]. …”

“…Some HHKs (called unorthodox HKs) also contain a histidine phosphotransferase domain (HPT). The P2CS database classified HKs into three major families, classical HKs, HHKs, and unorthodox HKs, as well as the CheA-like HKs [22,26]. There are also hybrid RRs (HRRs) containing HK domains [27,28].…”

“…There are also hybrid RRs (HRRs) containing HK domains [27,28]. There are 35 different families of RRs based on different types of output domains [22,26,29]. Approximately 50% of the activated RRs undergo homodimerization involving conserved residues at their receiver domains, and dimer formation is required for interaction with the regulatory regions of the target genes, which may contain tandem and inverted repeats [16,22].…”