A cyanobacterial gene that interferes with the phosphotransfer signal transduction involved in the osmoregulatory expression of ompC and ompF in Escherichia coli.

Synechococcus sp. PCC7942 is a phototrophic cyanobacterium. In this study we cloned a Synechococcus gene that has a striking effect on the production of the Escherichia coli outer membrane proteins, OmpC and OmpF, provided that this gene was introduced by a multicopy plasmid into the heterologous cells. This multicopy gene in E. coli cells was able to specifically shut off the production of both OmpC and OmpF at the level of transcription. The nucleotides were sequenced for this gene, named sis, and its gene product was purified from E. coli to near homogeneity. A computer-aided search found that the deduced amino acid sequence consisting of 138 residues is novel, with no significant similarity to any other protein in the databases. Since the transcription of ompC and ompF is regulated by the regulatory factors EnvZ and OmpR, through phosphotransfer signal transduction, we explored the inhibitory effect of Sis in various genetic backgrounds as to envZ and ompR. In particular, the inhibitory effect of Sis was observed even in an DeltaenvZ background, but was not observed in a certain background in which the ompC and ompF transcription was supported by a mutant OmpR that can function in a phosphorylation-independent manner. These results suggested that the EnvZ kinase may not be the direct target of Sis, but rather that the process(es) concerning the phosphorylation and/or dephosphorylation of the OmpR protein may be affected by Sis. However, no direct effect of Sis was seen in an in vitro OmpR-phosphorylation assay with the purified OmpR and Sis proteins. Based on these results, possible functions of Sis are discussed with special reference to the phosphotransfer signal transduction in E. coli.