THE CHROMOSOME OF SALMONELLA-PARATYPHI A IS INVERTED BY RECOMBINATIONBETWEEN RRNH AND RRNG

Salmonella paratyphi A, a human-adapted bacterial pathogen, causes par atyphoid enteric fever. We established the genome map of strain ATCC 9 150 by the use of four endonucleases,XbaI, I-CeuI, AvrII (= BlnI), and SpeI, which generated 27, 7, 19, and 38 fragments, respectively; the sum of the fragments in each case indicates a genome size of ca. 4,600 kb, With phage P22, we transduced Tn10 insertions in known genes from Salmonella typhimurium LT2 to S. paratyphi A ATCC 9150 and located th ese insertions on the S. paratyphi A chromosome through the XbaI and A vrII sites in Tn10 and through the increased size of the SpeI fragment bearing a Tn10. Compared with the maps of other Salmonella species, t he S. paratyphi A genomic map showed two major differences: (i) an ins ertion of about 100 kb of DNA between rrnH/G and proB and (ii) an inve rsion of half the genome between rrnH and rrnG, postulated to be due t o homologous recombination between the rrn genes, We propose that duri ng the evolution of S, paratyphi A, the first rearrangement event tvas the 100-kb insertion, which disrupted the chromosomal balance between oriC and the termination of replication, forcing the rrnH/G inversion to restore the balance. The insertion and the inversion are both pres ent in all 10 independent wild-type S. paratyphi A strains tested.