rpe, a cis-acting element from the strA region of the Haemophilus influenzae chromosome that makes plasmid establishment independent of recombination

Abstract

Plasmids that share homology with the Haemophilus influenzae chromosome transform wild-type cells more efficiently than they transform recombination-defective mutants. A 5.2-kilobase-pair chromosomal fragment containing the strA gene of H. influenzae was found to promote efficient plasmid establishment in recombination-defective mutants. A cis-acting element in the insert, called rpe for rec-less plasmid establishment, promoted plasmid transformation in rec-1 and rec-2 mutants without suppressing the recombination defects of these strains. The rpe locus increased plasmid transformation in wild-type cells without interfering with the pathway of plasmid establishment that is dependent on recombination functions.Competent Haemophilus influenzae cells can acquire plasmids by transformation. If the transforming plasmid and the recipient chromosome share no homology or if a recombination-deficient strain is transformed by a recombinant plasmid, establishment occurs at a low frequency (10, 16). When recombination between the plasmid and the chromosome is possible, the frequency of plasmid establishment increases by up to several orders of magnitude (16). The mechanism by which recombination functions promote plasmid establishment is not known. Pairing between homologous segments of the incoming plasmid and the recipient chromosome might promote the repair of partially degraded plamids and protect them from further breakdown (14, 16).We isolated a recombinant plasmid (called pKLT1) that contained the strA gene within a 5.2-kilobase (kbp) chromosomal fragment. A cis-acting locus adjacent to the strA gene (called rpe for rec-less plasmid establishment) was found that caused plasmid establishment to be independent of the recombination ability of the recipient cell. The rpe locus did not promote establishment by suppressing the recombination defects associated with rec-l and rec-2 mutants.