Abstract
The structure of the circular prophage genome of ΦH varies with high frequency in single colony progeny of the defective lysogen Halobacterium halobium R1-3. As in linear ΦH 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 R1-L, which contains this plasmid, is immune to phage infection. A phage variant, ΦHL1, is able to grow on R1-L and carries an insertion of 1 kilobase pair in its L segment. ΦHL1 does not grow on normal lysogens. This shows that the plasmid confers to R1-L only part of the immunity of normal lysogens.
Keywords: archaebacteria, structural variability, phage variant, insertion element, recombination
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