Genomic rearrangements (duplications and inversions) in enteric bacteria such as Salmonella enterica serovar Typhimurium LT2 and Escherichia coli K12 are frequent (10-3 to 10-5) in culture, but in wild-type strains these genomic rearrangements seldom survive. However, inversions commonly survive in the terminus of replication (TER) region, where bidirectional DNA replication terminates; nucleotide sequences from S. enterica serovar Typhimurium LT2, S. enterica serovar Typhi CT18, E. coli K12, and E. coli O157:H7 revealed genomic inversions spanning the TER region. Assuming that S. enterica serovar Typhimurium LT2 represents the ancestral genome structure, we found an inversion of 556 kb in serovar Typhi CT18 between two of the 25 IS200 elements and an inversion of about 700 kb in E. coli K12 and E. coli O157:H7. In addition, there is another inversion of 500 kb in E. coli O157:H7 compared with E. coli K12. PCR analysis confirmed that all S. enterica serovar Typhi strains tested, but not strains of other Salmonella serovars, have an inversion at the exact site of the IS200 insertions. We conclude that inversions of the TER region survive because they do not significantly change replication balance or because they are part of the compensating mechanisms to regain chromosome balance after it is disrupted by insertions, deletions, or other inversions.

目的通过基因组结构分析对临床分离的Eseherichia coil（E.coli）进行分型, 并探讨分型与临床疾病的关系。方法取临床不同疾病病人的痰、尿、血、分泌物等标本，分离E.coil。用I-Ceu I酶切全基因组DNA，用脉冲场凝胶电泳分离DNA片段后，根据酶切图谱的异同进行分型。结果从临床上分离的64株E.coli中，62株有7个I-Ceu I酶切位点，2株有8个I-Ceu I酶切位点。菌株间I-Ceu I酶切图谱差异明显。这些菌株根据基因组结构的差异分为32个型，分型与疾病之间的对应关系分散。结论临床分离的E.coli基因组结构存在多样性，其与临床疾病之间的关系有待进一步探讨。

Salmonella paratyphi A, a human-adapted bacterial pathogen, causes paratyphoid enteric fever. We established the genome map of strain ATCC 9150 by the use of four endonucleases, XbaI, I-CeuI, AvrII (5 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 these insertions on the S. paratyphi A chromosome through the XbaI and AvrII sites in Tn10 and through the increased size of the SpeI fragment bearing a Tn10. Compared with the maps of other Salmonella species, the S. paratyphi A genomic map showed two major differences: (i) an insertion of about 100 kb of DNA between rrnH/G and proB and (ii) an inversion of half the genome between rrnH and rrnG, postulated to be due to homologous recombination between the rrn genes. We propose that during the evolution of S. paratyphi A, the first rearrangement event was 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 present in all 10 independent wild-type S. paratyphi A strains tested.

Partial digestion with I-CeuI, which digests bacterial DNA at the gene coding for the large subunit rRNA, established the rrn genomic skeleton (the distance in kb between rRNA operons) in 56 strains of Salmonella, from Salmonella Reference B (SARB) set. All had seven I-CeuI sites, indicating seven rrn operons. The order of I-CeuI fragments was ABCDEFG in S. typhimurium LT2 and in 31 other species, mostly host-generalists; in S. typhi, S. paratyphi C, S. gallinarum, and S. pullorum (host-specialized species), these fragments are rearranged, due to homologous recombination between the rrn operons. Rearrangements, such as inversions and translocations not involving the rrn operons, are rare. I-CeuI fragments of some species are larger than the norm, suggesting the insertion of unique blocks of DNA by lateral transfer from other species.

The enzyme I-CeuI, encoded by a class I mobile intron inserted in the gene for 23S rRNA in Chlamydomonas eugamatos, cleaves a specific 19-bp sequence in this gene. This sequence is present only in the seven genes for rRNA in Salmonella typhimurium and Escherichia coli. Partial digestion with I-CeuI of DNA from 17 wild-type strains of S. typhimurium indicates that the chromosome of these strains is strongly conserved, for the digestion products closely resemble those of strain LT2. The lengths and order of chromosomal segments are conserved in 15 of the strains; 2 show some rearrangements. XbaI digestion indicated heterogeneity without revealing the genomic structure. Because of conservation of I-CeuI sites in genes for rRNA and conservation of the number and locations of these genes, I-CeuI provides an excellent tool for the rapid examination of the chromosomes of related species of bacteria; differences in the fingerprints indicate the occurrence of chromosomal rearrangements such as insertions or inversions.