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TraJ is the positive activator of the major transfer operon in the F plasmid of Escherichia coli that counteracts H-NS silencing at the main transfer promoter (P(Y)). Multiple sequence alignment revealed a putative PAS (Per-ARNT-Sim) domain that might be involved in sensing redox potential or energy levels in the cell. This domain, which contains a conserved PXCXR motif along with a C(X)(9-10)CR/N/K motif of variable position, was identified within the N-terminal region of TraJ orthologues including F TraJ. The 5 cysteine residues in F TraJ were changed to serine to give protein with single or multiple substitutions. Single C to S substitutions had little effect on mating efficiency (ME), whereas cumulative substitutions from the N- to the C-termini (2CS to 5CS) gradually reduced ME to undetectable levels. F TraJ was able to bind to Fe (III) on an affinity sorbent column. This feature was severely impaired for the 5CS mutant. Thus, the cysteine residues within the PAS domain could be the part of a metal-containing redox centre that plays a key role in the transcriptional activation of the P(Y) operon by TraJ.

Recent evidence suggests that bacterial conjugation is ubiquitous in the bacterial world and that DNA transfer between different genera-kingdoms is possible. It has also been demonstrated that many bacterial gene transfer systems resemble each other at the molecular level and that others are a blend of two or more of these systems. Thus, in the absence of a sexual cycle, bacterial conjugation, along with bacteriophages, transposons, and natural transformation systems, forms a potent force for gene dissemination and repair in the eubacteria and simple eukaryotes.

Pseudomonas aeruginosa strains PAO and PAK bear polar pili which are flexible filaments having a diameter of 6 nm and an average length of 2500 nm. Both types of pili are retractile and promote infection by a number of bacteriophages. The present communication describes the partial biochemical characterization of PAO pili isolated from a multipiliated nonretractile mutant of PAO. The observed properties are compared to those of PAK pili which were characterized previously. PAO pili were found to contain a single polypeptide subunit of 18 700 daltons. This is similar to PAK pili which contain a single polypeptide of 18 100 daltons. The amino acid composition of PAO pilin was also similar to that of PAK pilin. Neither protein contained phosphate or carbohydrate residues and both were found to contain N-methylphenylalanine at the amino terminus. Sequencing of 20 amino acid residues at the amino terminal end of PAO pilin revealed the sequence to be identical with that of PAK pilin, while tryptic peptide analyses of PAO and PAK pilin indicated that the two proteins probably contain a number of homologous regions within the polypeptide. It was concluded that PAO and PAK pili were closely related structures.