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

The structure of the circular prophage genome of PhiH varies with high frequency in single colony progeny of the defective lysogen Halobacterium halobium R(1)-3. As in linear PhiH DNA, a segment flanked by two 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 R(1)-L, which contains this plasmid, is immune to phage infection. A phage variant, PhiHL1, is able to grow on R(1)-L and carries an insertion of 1 kilobase pair in its L segment. PhiHL1 does not grow on normal lysogens. This shows that the plasmid confers to R(1)-L only part of the immunity of normal lysogens.