An immune strain of Halobacterium halobium carries the invertible L segment of phage ΦH as a plasmid

Abstract

The structure of the circular prophage genome of .PHI.H varies with high frequency in single colony progeny of the defective lysogen H. halobium R1-3. As in linear .PHI.H DNA, a segment flanked by 2 copies of the insertion element ISH1.8 is inverted frequently. This L segment can also circularize to a plasmid of 12 kilobase pairs with simultaneous loss of the remaining phage DNA. Strain R1-L, which contains this plasmid, is immune to phage infection. A phage variant, .PHI.HL1, is able to grow on R1-L and carries an insertion of 1 kilobase pair in its L segment. .PHI.HL1 does not grow on normal lysogens. This shows that the plasmid confers to R1-L only part of the immunity of normal lysogens.