Preface to Two-Component Systems in Bacteria
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Two-component (TCS) systems are the predominant signal transduction devices of Eubacteria, but in a limited number they are also present in Archaea and some Eukaryotes. Most TCS have a relatively simple modular architecture which generally requires at least four domains (signal input, transmitter, receiver, and signal output domains) distributed among two proteins, the sensory histidine kinase and the response regulator. During evolution, their modular architecture has enabled the employment of these systems in many different cellular processes, which need to be regulated either by extracellular of intracellular stimulus changes, by simply exchanging signal input and output modules flanking the conserved signal transducing transmitter and receiver modules. In consequence, these systems are able to respond to an extremely broad spectrum of diverse stimuli and control a vast array of cellular functions. For instance, some TCS are involved in the regulation of very basic metabolic processes such as the energy metabolism, but also in important basic features of bacterial life such as cell division and differentiation. Moreover, TCS are also engaged in special requirements of bacteria with a dedicated life style such as pathogens or symbionts which need to interact with host species and to control virulence or symbiosis factors. Highlighting the enormous impact of TCS on the biology of Eubacteria, genome sequencing revealed the presence of TCS in virtually all of them. Some environmental bacteria with quite large genomes may encode hundreds of TCS proteins. On the other hand, bacteria living in niches believed to be quite constant regarding the physicochemical conditions such as obligate intracellular pathogens or symbionts encode a strongly reduced number of TCS or, in rare cases, they are lacking them entirely.
Many basic questions about the molecular function of TCS including structure-function relationships of the signalling domains and signal transduction fidelity have been characterized since their discovery in the late 80s of the last century. Nevertheless, many basic features remain to be investigated. For instance, a major issue still concerns the identification of the stimuli perceived by many TCS and the way how these stimuli are actually sensed. Moreover, the interconnection of different TCS by cross-talk or via so-called connector proteins which allow the establishment of complex regulatory networks is a challenging task for future research.
Since their discovery thousands of articles were published about many aspects of TCS and, accordingly, it is impossible to handle these systems in a single book in a comprehensive way. Therefore, several novel aspects of signal transduction mechanisms by TCS and of their biological function in different bacteria have been selected. The book starts with a chapter illustrating bioinformatics approaches to describe and characterize TCS. Further issues discussed involve novel insights in signal perception mechanisms by TCS and the role and mode of action of atypical and essential TCS. Finally, several examples of TCS engaged in cellular differentiation processes and bacterial pathogenesis and symbiosis are described. In conclusion, the book represents a compendium of current TCS research and hopefully may help to identify important future research activities.
Finally, we would like to thank all authors for their great collaboration during preparation of this book.
Roy Gross and Dagmar Beier
