Cell-surface signalling in pseudomonas

Abstract

Next to the two-component and quorum sensing systems, cell-surface signalling (CSS) has been recently identified as an important regulatory system in Pseudomonas. CSS senses signals from outside the cell and transmits them into the cytoplasm. This regulatory system generally consists of three components, an extracytoplasmic function (ECF) sigma factor in the cytoplasm, a sigma factor regulator (or anti-sigma factor) located in the cytoplasmic membrane, and an outer membrane receptor. The outer membrane receptor belongs to the TonB-dependent receptor family, which are mostly involved in the transport of iron-siderophore complexes across the outer membrane. To accomplish this task these receptors need to be energized by the TonB-ExbBD complex. Fluorscent pseudomonads are entangled in a fierce competition for iron. To fulfil their iron requirements, most species produce the yellow-green siderophores pyoverdine, but they are also highly specialized in the utilization of heterologous siderophores. For this they have an impressive amount of different TonB-dependent receptors that are not only produced under low iron conditions but are also dependent on the presence of the heterologous siderophores in the direct surrounding of the bacterium. CSS systems are responsible for this extra level of regulation. This signal transduction pathway starts with binding of the inducing signal, which is usually the iron-siderophore complex, to the TonB-dependent receptor. The signal is then transduced to the cytoplasmic membrane protein, which leads to the activation of ECF sigma factor. The activated sigma factor directs the RNA polymerase to the promoter region of gene(s) under control of the CSS system. One of the genes that is normally activated by the CSS pathway is the one encoding the TonB receptor, which is used for both the transport of the iron siderophore complex and the CSS signal transduction pathway that upregulates its synthesis. In addition, also other genes can be upregulated such as periplasmic transport system for the siderophore complex. In this chapter we review current knowledge of the better characterized CSS system of pseudomonads and discuss the role that other not yet experimentally analyzed CSS systems may have.