Ec. Becker et Rj. Meyer, ACQUISITION OF RESISTANCE GENES BY THE INCQ PLASMID R1162 IS LIMITED BY ITS HIGH COPY NUMBER AND LACK OF A PARTITIONING MECHANISM, Journal of bacteriology, 179(18), 1997, pp. 5947-5950
R1162 is a representative member of the broad-host-range IncQ group of
multicopy plasmids. Lower-copy-number derivatives of R1162 were const
ructed in vitro and shown to be unstable, indicating that partitioning
of plasmid copies at cell division is due to random distribution and
not to an active partitioning mechanism, However, the normal copy numb
er of R1162 reduces cell fitness during growth in broth and favors the
emergence of unstable, lower-copy-number variants. As a result, plasm
id-borne antibiotic resistance genes active at a low copy number event
ually result in plasmid loss during periods of no selection. We argue
that the maintenance of R1162 in a population requires a gene that is
selected only at high levels. This reduces the potential for acquiring
genes from other R factors and could explain the limited variety of a
ntibiotic resistance genes contained by naturally occurring IncQ plasm
ids.