An oligomer probe was designed to detect the presence of a putative phoB gene in the genome of the marine, phycoerythrin-containing cyanobacterium Synechococcus sp. WH7803. A 2.2 kb PstI fragment, identified using this probe, was cloned and the complete nucleotide sequence determined. The fragment contained two open reading frames encoding polypeptides which display all the sequence features expected of the response regulator and histidine protein kinase elements of a two component sensory system. Northern analysis confirmed that transcription of these genes was induced by phosphate limitation. On the basis of the sequence similarities and the regulation of their transcription by the availability of inorganic phosphate (Pi) these open reading frames were designated as phoB and phoR, respectively.