Abstract
Two Hfr strains which had recieved the thymine requiring mutation of strain 15T-, but the colicinogenic factor were isolated. Mating experiments with these Hfr's revealed that thymine starvation of Hfr cells inhibits chromosome transfer. Uracil and histidine double starvation in Hfr cells, rather than being inhibitory, protects them from the spontaneous interruption of the process. The inhibitory effect of thymine starvation is differentially active, the more distant from the origin of an Hfr chromosome a given genetic marker is located, the more strongly transfer of that marker is inhibited. This inhibitory effect is (at least, partially) reversible.
