O0 and strong-polar mutations in the gal operon are insertions

Summary Three dg phages carrying strong-polar mutations in the gal operon are denser than the corresponding phages carrying the wildtype gal operon or reversions of the mutations to the Gal⁺ phenotype. The latter phages have the same density. It is concluded that these strong-polar mutations are insertions of DNA into the gal operon.The amount of inserted DNA is different in the three mutations and is calculated to be 450, 1,080 and 1,800 nucleotide pairs respectively.The strong-polar phenotype is also found in a mutant supplied by A. Taylor which carries a Mu-1 phage integrated into the transferase gene.