The state of phage psi DNA in lysogenic cells of Agrobacterium tumefaciens.
Phage psi a temperate phage of Agrobacterium tunefaciens is mutagenic, grows poorly in strains which contain a pTi plasmid, and can be transferred by conjugation (D. Expert and J. Tourneur, 1982, J. Virol., 42, 283-291). In an attempt to explain these unusual properties we undertook a study of the state of psi DNA in the virion, during the lytic cycle, and in lysogenic cells. In the virions, the phage genome was found to consist of circularly permuted molecules. Covalently closed circular forms of these molecules could be detected in lytically infected cells. These appeared late in phage infection, accumulated in pTi-containing cells. The same circular forms were found in lysogens but they could not correspond to the permanent state of phage psi because they were present at less than one copy per cell. Restriction enzyme analysis and Southern blotting revealed the presence of a psi prophage integrated in the host chromosome. The site of integration in the chromosome was unique and the same in all lysogens tested. The results implied that the integration involves a circular form of psi DNA. The bearing of these findings regarding the properties of phage is discussed.
Expert
D
D
Laboratoire de Biologie cellulaire, CNRA 78000 Versailles, France.
Riviere
F
F
Tourneu
J
J