Restriction in Matings of Escherzchza Coli

RESTRICTION in Escherichia coli limits the acceptance of genetic elements such as bacteriophage, sex-factor, and donated bacterial chromosome. These extrinsic genetic elements carry a specificity determined by their last host. When the specificity differs from that of the new potential host, the genetic element is restricted by the new host cell. Restriction is manifest by low efficiency of plating for phage, diminished acceptance of sex-factors or chromosomal markers, or by reduction in linkage of markers known to be closely associated. Modification is a host-induced superimposition of a new specificity on these genetic elements and a conferring of this new specificity on their replicas. Once modified, the formerly restricted genetic elements can be freely accepted by other cells that have specificities either identical with, or similar to, the modifying host. Restriction of the bacterial chromosome has been shown in matings between an E. coli K-12 nonlysogenic donor and an E. coli K-12 recipient lysogenic for prophage P1 (ARBER 1962). In such a cross, the frequency of recombinants is reduced compared to frequencies in crosses where neither parent is lysogenic. These results have been confirmed and extended by PITTARD (1964) who demonstrated that in a cross between a P1-sensitive donor and a P1-lysogenic recipient the frequency of selected recombinants and the linkage between markers is diminished. A more complete analysis of modification and restriction in the bacterial conjugation system has been presented by BOYER (1964). Crosses between E. coli K-12 Hfr and B/r F-, give a frequency of recombination about 10 to 15 times lower for the selected marker, compared to a cross in which K-12 Fis used as a recipient. BOYER shows that a region on the E. coli map between pi1 and thr controls the mating response between K-12 and B/r. Interrupted matings of the restricted K-12 X B/r cross gives surprisingly different results from the unrestricted K-12 x K-12 cross. Proximal markers, known to enter about 8 min after mating, do not appear in the recombinants until after 30 min in the restricted cross. The frequency of unselected markers is reduced about one half in the restricted cross compared to the unrestricted cross. Restriction and modification of other genetic elements, such as bacteriophage lambda, F-lac, F-gal and sex-factor F1 , are found to be controlled by the same genetic region that controls the mating response