Functional analysis of a Campylobacter jejuni alkaline phosphatase secreted via the Tat export machinery

Bacterial alkaline phosphatases (PhoA) hydrolyse phosphate-containing substrates to provide the preferred phosphorus source inorganic phosphate (Pi). Campylobacter jejuni does not contain a typical PhoA homologue but contains a phosphatase that is regulated by the two-component system PhosS/PhosR. Here we describe the characterization of the enzyme, its secretion pathway and its function in the bacterium's biology. Phosphatase assays showed that the enzyme utilizes exclusively phosphomonoesters as a substrate, requires Ca2+ for its activity, and displays maximum activity at a pH of 10. Gene disruption revealed that it is the sole alkaline phosphatase in C. jejuni. The protein contained a twin-arginine motif (RR) at its N terminus, typical of substrates of the Tat secretion system. Substitution of the twin-arginine residues showed that they are essential for enzyme activity. C. jejuni genome analysis indicated the presence of four ubiquitously expressed Tat components that may form a functional Tat secretion system as well as 11 putative Tat substrates, including the alkaline phosphatase (PhoACj) and the nitrate reductase NapA. Inactivation of tatC caused defects in both PhoACj and NapA activity as well as a reduction in bacterial growth that were all restored by complementation in trans with an intact tatC copy. The atypical overall features of the PhoACj compared to Escherichia coli PhoA support the existence in prokaryotes of a separate group of Tat-dependent alkaline phosphatases, classified as the PhoX family.