Transcription of the region encoding the ferric dicitrate-transport system in Escherichia coli: similarity between promoters for fecA and for extracytoplasmic function sigma factors

Abstract

We have analyzed the molecular mechanism of regulation of the ferric dicitrate transport system in Escherichia coli (Ec), by studying the transcription of the regulatory and structural genes under various environmental conditions, and by determining the location of their transcriptional start points and promoter regions. We report here that the main species observed in Northern hybridization analyses were a 2.5-kb mRNA, encoded by the outer membrane protein receptor gene fecA, and a 1.5-kb mRNA encoded by a region including the fecIR genes. The synthesis of the 2.5-kb fecA mRNA is regulated by both citrate and iron. Furthermore, transcription of fecA is dependent on the presence of FecI. The promoter region for the fecA mRNA, a likely site of action for FecI, is not related to the consensus promoter region for sigma 70 RNA polymerase in Ec K-12. However, it shows greatest similarity with promoters of genes regulated by a new sub-family of sigma factors, i.e., the extracytoplasmic function (ECF) sigma factors, which are associated with the expression of genes involved in extracytoplasmic functions, suggesting that FecI may act as a specialized sigma factor. We also show that the fecB,C,D,E transport genes are linked in operon fashion to fecA. Since the levels of the fecB,C,D,E RNAs are extremely low, as compared to the level of fecA mRNA, it is likely that processing from the 3' end must occur and stop near the end of fecA where a hairpin structure is located.