Technical Abstract: Two-component signal transduction systems play a vital role in the ability of bacteria to adapt to a changed environment by altering the gene expression in response to a wide variety of external stimuli. Here we report that the C. jejuni Cj0889 and Cj0890 genes form a two-component system as demonstrated by phosphorylation assays. This system is activated in response to phosphate starvation and is required for optimal growth in phosphate limiting media. At the amino acid level the Cj0889/Cj0890 two-component system is not homologous to the PhoB/PhoR two-component system present in many other organisms, therefore we have designated the Cj0889/Cj0890 system as PhoS/PhoRR. Microarray analysis and real-time RT-PCR experiments revealed that the PhoS/PhoRR system is not autoregulated, but regulates the transcriptional units of the pho regulon: pstSCAB (Cj0613-Cj0616), Cj0145, and the 7 genes contained in the operon Cj0727-Cj0733. Gel shift assays demonstrated that His-tagged PhoRR protein bound DNA fragments containing the promoter regions upstream of these three transcriptional units that all carried a highly conserved DNA sequence, which we named the pho box of C. jejuni (5’-GTTTCNAAAANGTTTC-3’). Functional analysis demonstrated that Cj0145 encoded a monomeric alkaline phosphatase homologous to Vibrio cholera PhoAvc was regulated by PhoS/PhoRR system. In conclusion our results show that C. jejuni carries a pho regulon that is regulated by the PhoS/PhoRR system, which is functional similar but unrelated to the classical PhoB/PhoR two-component systems.