Transformation and isolation of allelic exchange mutants of Chlamydia psittaci using recombinant DNA introduced by electroporation

To facilitate genetic investigations in the obligate intracellular pathogens Chlamydia, the ability to construct variants by homologous recombination was investigated in C. psittaci 6BC. The single rRNA operon was targeted with a synthetic 16S rRNA allele, harboring three nucleotide substitutions over 398 bp, which imparts resistance to kasugamycin (Ksm) and spectinomycin (Spc) and causes loss of one HpaI restriction site. A fourth, silent mutation was introduced 654 bp downstream in the beginning of the 23S rRNA gene. C. psittaci 6BC infectious particles were electroporated with various concentrations of circular or linearized plasmids containing different lengths of the rRNA region homologous to the chromosomal copy except for the four nucleotide substitutions. Ksm and Spc were added 18 h after inoculation onto confluent cell monolayers in the plaque assay. Resistant plaques were picked and expanded with selection 10 days later before collecting DNA for analysis by PCR, restriction mapping, sequencing, or Southern. Spontaneous resistance to Ksm and Spc was never observed in mock electroporated bacteria (frequency< 6.2 x 10(-9)). Conversely, double resistance and replacement of the 16S rRNA gene were observed when C. psittaci was electroporated with the recombination substrates. Highest efficiency was obtained with 10 mu g of circular vector prepared in a DNA methylase-deficient Escherichia coli (1.9 +/- 1.1 x 10(-6), n = 7). Coinheritance of the silent 23S rRNA mutation was seen in 46 of 67 recombinants analyzed, illustrating DNA exchange of up to 1,052 bp in length. These findings provide the first step toward genetic manipulation of Chlamydia.