A genetic factor on the DNA molecule capable of depressing the frequency of transformation at a linked site.

HE genetic factor responsible for the depression of the frequency of streptomycin-resistance transformations in Pneumococcus was first studied by GREEN (1959). He found that when Rx cells are exposed to a DNA transforming preparation (TP) containing genetic markers for erythromycin resistance and streptomycin resistance, approximately identical frequencies of transformation by both markers are observed. However, if strain Rz is exposed to the same DNA, the frequency of streptomycin-resistance transformations decreases. This difference between the two strains has been termed a “host specificity.” The specific depression of transformation of a streptomycinresistance marker has also been reported by STUY (1961 ). GREEN hypothesized that this specific depression is due to a genetic factor (depressor or dep) which is linked to the streptomycin-resistance marker and is usually transmitted in association with that marker when the latter is successfully integrated into the recipient genome. However, this conclusion was not completely justified since the experimental design permitted several alternative conclusions; it was possible that the depressor was akociated with both the erythromycin-resistance marker and the streptomycin-resistance marker, or even with the erythromycinresistance marker alone. GREEN suggested that the depressor has its effect at the time the transferred markers are genetically integrated into recipient genomes. He suggested that the depressor factor is possibly one that interferes with the recombination event. Since studies of heterospecific transformation ( SCHAEFFER 1956; ALEXANDER, LEIDY and HAHN 1954) have shown that a certain homology between the DNA of the recipient and the transforming DNA is necessary for efficient transformation, GREEN suggested that the depressor is a portion of a DNA molecule, in the TP, which lacks homology with a corresponding region of the same DNA molecule in the recipient Rz strain, but which is homologous to this region in the Rx strain. Consequently, the depressor lowers the frequency of transforma-