Chlamydial recombination

Genome sequence analyses suggest that chlamydial chromosomes are very similar within and largely syntenous between species, with minimal recent contributions from other organisms [1-3]. There is only one example of recent chlamydial acquisition of foreign DNA- the tet(C) island identified in our laboratory [17,18].  However, accumulating sequence data indicates that chlamydiae are actively recombinogenic within a specific genus.  Early work by several investigators identified omp1 variants containing 5’ and 3’ ends that encode protein sequences from different classical serovars [4, 7-9].  Dean et al. also showed that recombination may occur in the pmp genes and at other locations in the chromosome [10, 11].  The advent of genome sequencing allowed the identification of additional chlamydial recombination events [5, 6].  Genomic analyses also led to the identification of a region of the chromosome termed the plasticity zone, in which significant genetic differences occur, and how these variations are associated with unique biological properties of different chlamydial strains [5,12,13].   

Recent and exciting work by Demars and colleagues [14, 15] demonstrates that C. trachomatis can undergo stable intraspecies horizontal gene transfer during growth in vitro.  Using a strategy parallel to that of Demars et al. [15], we mixed differently antibiotic resistant strains of C. trachomatisC. suis and C. muridarum, and screened for recombinants with appropriate combinations of antibiotics [16].  For example, a cross between C. trachomatis L2 encoding ofloxacin resistance and C. trachomatis 6276/J encoding rifampin resistance was screened with ofloxacin and rifampin.  In these experiments, crosses transferring each antibiotic were generated within and among these species, and progeny from one cross could be used as parentals in a different cross. Finally, nucleic acid sequence analysis of progeny clones showed that the donor sequence for the resistance gene was found within the selected progeny.   

We examined whether tetracycline resistance carried by Chlamydia suis could be mobilized among these bacteria. We hypothesized that these crosses might be possible because the C. suisinclusion fuses with C. trachomatis inclusions and, therefore, these bacteria can develop within a single fused inclusion. Subsequent experiments demonstrated that C. suis and C. trachomatisresistant to both antibiotics were recovered from the crosses [16].  The resistant phenotype was stable both in the presence and absence of the antibiotic. PCR analysis confirmed the appropriateomp1 genotype and that the tet(C) allele was found in each progeny bacteria.  These studies were expanded to include both clinical and laboratory strains of C. trachomatis, and different strains ofC. suis,and C. muridarum.  In general the results discussed above were repeated- recombination was routinely observed in strains that occupy the same inclusion within cells.  In our experiments, recombination detectable but less common in crosses between C. trachomatis strains that do not form fusogenic inclusions. We have never observed recombination between the more distantly related guinea-pig-tropic C. caviae (strain GPIC) and any of the other tested strains.



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