Abstract
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 IS
200
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 IS
200
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.