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Chapter 20 : Conjugal Transfer of Chromosomal DNA in Legionella pneumophila
This chapter examines whether a chromosomal conjugation system exists in Legionella pneumophila. Mating experiments between K6 (approximately 2.6 X 109 CFU) and Chicago-2S (approximately 8.5 X 109 CFU) typically yielded 103 Kmr Smr LacZ+ recombinants. All recombinants tested (100/100) belonged to serogroup 6, which is the same serogroup as strain Chicago-2S. This result suggested that the donor was strain K6 and the recipient was strain Chicago-2S. The mechanism of the DNA transfer was most consistent with conjugal transfer. To genetically examine the DNA transferred from K6 to Chicago-2S, genotyping by arbitrarily primed PCR (AP-PCR) and pulsed-field gel electrophoresis (PFGE) were performed among strains K6, Chicago-2S, and their transconjugants. The restriction fragment patterns of the transconjugants were distinct from the pattern exhibited by K6 and were similar to that exhibited by Chicago-2S. The transfer of DNA from the chromosome of strain K6 to Chicago-2S could occur by a variety of mechanisms. Transposition would presumably transfer only the transposition sequences to random sites that would differ in different recombinants. Researchers showed that a thymine auxotroph of L. pneumophila was repaired by plasmid R68.45-mediated chromosomal mobilization of a prototrophic donor strain. Therefore, if L. pneumophila had acquired an onT by integration of these conjugative elements, it would become selfmobilizable like the Hfr of Escherichia coli.