Molecular characterization, nucleotide sequence, and expression of the fliO, fliP, fliQ, and fliR genes of Escherichia coli

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Abstract
The fliL operon of Escherichia coli contains seven genes that are involved in the biosynthesis and functioning of the flagellar organelle. DNA sequences for the first three genes of this operon have been reported previously. A 2.2-kb PstI restriction fragment was shown to complement known mutant alleles of the fliO, fliP, fliQ, and fliR genes, the four remaining genes of the fliL operon. Four open reading frames were identified by DNA sequence analysis and correlated to their corresponding genes by complementation analysis. These genes were found to encode very hydrophobic polypeptides with molecular masses of 11.1, 26.9, 9.6, and 28.5 kDa for FliO, FliP, FliQ, and FliR, respectively. Analysis of recombinant plasmids in a T7 promoter-polymerase expression system enabled us to identify three of the four gene products. On the basis of DNA sequence analysis and in vivo protein expression, it appears that the fliP gene product is synthesized as a precursor protein with an N-terminal signal peptide of 21 amino acids. The FliP protein was homologous to proteins encoded by a DNA sequence upstream of the flaA gene of Rhizobium meliloti, to a gene involved in pathogenicity in Xanthomonas campestris pv. glycines, and to the spa24 gene of the Shigella flexneri. The latter two genes encode proteins that appear to be involved in protein translocation, suggesting that the FliP protein may have a similar function.