A mutation ereating an operon fusion to lacZ was isolated, which results in partial suppression of motility repression at high temperature. The insertional phenotype was called Tim designing temperature-independant motility. Genetic mapping revealed that the mutation is located at 51.5 min of E. coli linkage map. Further analysis with cloned DNA fragements indicates that the insertion lies in the region downstream of ackA gene to toward purF at 52.4 min. The insertion lacks phosphotransacetylase activity and exhibits acetate growth deficiency as well as monophotransacetylase. Therefore, it is called pta-1::Tn10-lacZ or pta-1.
The pta-1 mutation affects the chemotatic behavior of E.coli in several ways. In addition to its effect on swarming enhancement at high temperature, the mutation alters the normal swimming behavior such that the chemotatic ring on tryptone swarm plate is smaller and diffused even at optimal temperature. The insertion also increases swimming speed of motile cells. An experiment with flhDC::lacZ fusion, occuring at the top level of regulatory cascade, and the detection of flagellar synthesis with anti-flagellin anti-serum demonstrated that the pta-1 insertion significantly increase the expression of the whole flagellar regulon in comparsion with wild type. In contrast, a mutation in acetate kinase gene ackA reduces flhDC expression, thereby resulting in a poorly motile phenotype. The pta mutation is epistatic to the ackA mutation in terms of flagellar expression as tested in ackA pta double mutant. However, unlike the pta mutation the elevated level of flagellar expression in the ackA pta double mutant was insensitive to an externally added acetate in the medium.
The mutational effect of either ackA or pta on the flagellar expression was no longer observed when the mutant strains had an additional mutaion in ompR gene, encoding a transcriptional regulator that is phosphorylated in response to a change in medium osmolarity. These results suggest that the transcription of flagellar master operon is modulated by internal level of acetyl phosphate through the OmpR protein as part of the global regulatory network. In addition, the negative effect of an increase in medium osmolarity, that enhances OmpR phosphorylation, on flhDC expression was not found in ompR mutant. OmpR protein binds to the DNA fragment containing the flhDC promotor and its affinity is increased with phosphorylation by acetyl phosphate. The DNase I footprinting revealed the regions of the flhDC promotor protected by OmpR in the presence or absence of phosphorylation. Our results imply that the phospho-OmpR generated by either osmolarity change or internal level of acetyl phosphate negatively regulates the expression of flagella.
