Aerobic regulation of the Escherichia coli tonB gene by changes in iron availability and the fur locus.

The tonB gene is required for the transport of several different iron-siderophore complexes across the Escherichia coli outer membrane. In this study, transcriptional regulation of the tonB gene was investigated by using three different tonB-lacZ fusions to monitor tonB expression under aerobic conditions and in the presence of a wild-type tonB gene. Prior work by other laboratories suggests that tonB is expressed at low constitutive levels regardless of changes in iron availability or the fur locus. In contrast, these data show that tonB transcription is repressed threefold by growth in the presence of FeCl3 compared with growth in the presence of the iron chelator dipyridyl and that this repression requires the fur locus. A 168-base-pair DNA fragment carrying the tonB promoter was sufficient for the observed transcriptional regulation. In addition, the tonB gene appeared to have a substantially stronger promoter than previously recognized. The inability of other laboratories to detect tonB transcription regulation appears to be due to the extremely slow growth of iron-starved tonB strains and the use of Mu d1(lac Apr)- or lambda plac Mu53-generated fusions that encode a thermolabile TrpA-LacZ hybrid protein. The data also suggest that the previously reported growth phase regulation of tonB occurs only in media with intermediate levels of available iron and is due to iron starvation-induced derepression as the culture approaches stationary phase.