The origin of plasmid DNA transfer during bacterial conjugation

Mutations in the trfA replication gene of the broad-host-range plasmid RK2 result in elevated plasmid copy numbers

Mutated forms of trfA, the replication protein gene of plasmid RK2, that support a minimal RK2 origin plasmid in Escherichia coli at copy numbers up to 23-fold higher than normal have been isolated. Six such high-copy-number (copy-up) mutations were mapped and sequenced. In each case, a single base transition led to an amino acid substitution in the TrfA protein primary sequence. The six mutations affected different residues of the protein and were located within a 69-base-pair region encoding 24 amino acids. Dominance tests showed that each of the mutants can be suppressed by wild-type trfA in trans, but suppression is highly dependent on the amount of wild-type protein produced. Excess mutant TrfA protein provided in trans significantly increased the copy number of RK2 and other self-replicating derivatives of RK2 that contain a wild-type trfA gene. These observations suggest that the mutations affect a regulatory activity of the TrfA replication protein that is a key factor in the control of initiation of RK2 replication.

A protein covalently bound to ColE1 DNA

ColE1 DNA was isolated from Escherichia coli as a relaxation complex of supercoiled DNA and proteins. Treatment of the complex with either protein-denaturing agents (SDS, phenol etc.) or proteolytic enzymes converted the supercoiled DNA to an open-circular form (relaxation). The relaxation complex was separately labelled in vivo with [3H]Leu or [14C]Leu, [35S]Met or (32P)phosphate and extensively purified. Complete hydrolysis of the relaxed complex with DNase I and P1 nuclease produced a 36-kDa protein which, we believe, is covalently bound to ColE1 DNA. On the other hand, the relaxed complex was treated with tosylphenylalanylchloromethane-treated-trypsin and the DNA-peptide(s) produced was (were) isolated and digested with the nucleases as above. The resulting nucleotidylpeptide(s) was (were) isolated by DEAE-Sephadex chromatography. The only 5'-dCMP was released from the nucleotidylpeptide(s) by snake venom phosphodiesterase treatment. O-Phosphoserine was found in acid hydrolysates of the DNA-peptide(s). We suggest that in the relaxation event the 36-kDa protein becomes covalently linked to ColE1 DNA via a phosphodiester bond between dC and the serine residue.

DnaA protein is required for replication of the minimal replicon of the broad-host-range plasmid RK2 in Escherichia coli

The minimal origin of replication of the broad-host-range plasmid RK2 has two potential recognition sequences for the DnaA protein of Escherichia coli. DNA transfer by transformation into a dnaA-null mutant of E. coli showed that DnaA protein is needed for replication or maintenance of mini-RK2. We isolated and purified DnaA protein as a chimeric protein, covalently attached to a piece of collagen and beta-galactosidase. The hybrid protein specifically bound to restriction fragments from the oriV region of RK2, which contained the two dnaA boxes. Deletion of the second dnaA box inactivated the origin and abolished the binding of the hybrid protein to the DNA fragment that had suffered the deletion. When the second dnaA box was replaced with an EcoRI linker of identical length, origin activity was restored. Binding experiments showed that the linker provided a weak dnaA box. An alternative explanation was that the linker restored proper spacing between sequences on either side of the deleted box, thus restoring origin activity.

IS21 insertion in the trfA replication control gene of chromosomally integrated plasmid RP1: a property of stable Pseudomonas aeruginosa Hfr strains

Broad host range IncP-1 plasmids are able to integrate into the chromosome of gram-negative bacteria. Strains carrying an integrated plasmid can be obtained when the markers of a temperature-sensitive (ts) plasmid derivative are selected at non-permissive temperature; in this way Hfr (high frequency) donor strains can be formed. The integrated plasmids, however, tend to be unstable in the absence of continuous selective pressure. In order to obtain stable Hfr donor strains of Pseudomonas aeruginosa PAO, we constructed a derivative of an RP1 (ts) plasmid, pME134, which was defective in the resolvase gene (tnpR) of transposon Tn801. Chromosomal integration of pME134 was selected in a recombination-deficient (rec-102) PAO strain at 43 degrees C. Plasmid integration occurred at different sites resulting in a useful set of Hfr strains that transferred chromosomal markers unidirectionally. The tnpR and rec-102 mutations prevented plasmid excision from the chromosome. In several (but not all) Hfr strains that grew well and retained the integrated plasmid at temperatures below 43 degrees C, the insertion element IS21 of RP1 was found to be inserted into the trfA locus (specifying an essential trans-acting replication function) of the integrated plasmid. One such Hfr strain was rendered rec+; from its chromosome the pME134::IS21 plasmid (= pME14) was excised and transferred by conjugation to Escherichia coli where pME14 could replicate autonomously only when a helper plasmid provided the trfA+ function in trans. Thus, it appears that trfA inactivation favours the stability of chromosomally integrated RP1 in P. aeruginosa.