Experimental genome evolution: large-scale genome rearrangements associated with resistance to replacement of a chromosomal restriction-modification gene complex
N. Handa et al., Experimental genome evolution: large-scale genome rearrangements associated with resistance to replacement of a chromosomal restriction-modification gene complex, MOL MICROB, 40(4), 2001, pp. 932-940
Type II restriction enzymes are paired with modification enzymes that prote
ct type II restriction sites from cleavage by methylating them. A plasmid c
arrying a type II restriction-modification gene complex is not easily repla
ced by an incompatible plasmid because loss of the former leads to cell dea
th through chromosome cleavage. In the present work, we looked to see wheth
er a chromosomally located restriction-modification gene complex could be r
eplaced by a homologous stretch of DNA, We tried to replace the PaeR71 gene
complex on the Escherichia coli chromosome by transducing a homologous str
etch of PaeR71-modified DNA, The replacement efficiency of the restriction-
modification complex was lower than expected. Some of the resulting recombi
nant clones retained the recipient restriction-modification gene complex as
well as the homologous DNA (donor allele), and slowly lost the donor allel
e in the absence of selection. Analysis of their genome-wide rearrangements
by Southern hybridization, inverse polymerase chain reaction (iPCR) and se
quence determination demonstrated the occurrence of unequal homologous reco
mbination between copies of the transposon IS3, It was strongly suggested t
hat multiple rounds of unequal IS3-IS3 recombination caused large-scale dup
lication and inversion of the chromosome, and that only one of the duplicat
ed copies of the recipient PaeR71 was replaced.