Characteristics of the transducing elements of bacteriophage φ80

The density of bacteriophage φ80 was measured by density gradient equilibrium centrifugation in CsCI solutions. This density was the same in phages derived from a lysogenic or a lytic growth cycle. The distribution of low-frequency transducing (LFT) particles of φ80 in a CsCl density gradient was shown to be much broader than that of plaque-forming particles. The transducing phages in the LFT lysates appear to transmit their characteristic densities, as well as definite try markers, to the φ80 dt particles in the high-frequency transducing lysates derived from them. The φ80 dt particles can be divided into two classes according to their requirement for helper phages in transduction. One class transduces more efficiently in the presence of active helper phage, while with the other class the active phage does not enhance or even reduces the transduction frequency. A new transducing phage type, which is not defective since it multiplies without help from the active phage, has been isolated. This phage, called φ80 pt (plaque-forming tryptophan), forms a small turbid plaque. A tryptophan auxotroph lysogenized by φ80 pt becomes try+, although it always segregates out some try− nonlysogenic cells. The buoyant density of φ80 pt in CsCI solution is greater than that of φ80, suggesting that a larger genetic fragment is carried in this phage.