Temperate and virulent forms of phage theta attacking Bacillus licheniformis

Summary

Clear-plaque phage ϑ_c, attacking bacitracin-producing strains of B. licheniformis, yields spontaneous temperate mutants at high frequency; the temperate mutants fall into several classes phenotypically different in plaque morphology and properties of lysogenised bacteria. The most common phenotype ϑ₃ has DNA restriction fragment patterns identical with those of the parent ϑ_c; some less common temperate forms, i.e. ϑ₁ and ϑ₂, produce different restriction fragment patterns, sugesting that a part of the original ϑ_c DNA has been reorganized or replaced by some foreign genetic material. The changed fragment pattern remains stable upon subsequent passaging of the phage or of the lysogenic bacteria. Neither class of temperate phage mutants gives clearplaque revertants at measurable frequency. Lysogenisation of bacteria with any class of temperate phage confers immunity to all temperate forms and to ϑ_c; virulent mutants ϑ_vir, which plate with 100% efficiency on lysogens for ϑ₁ and ϑ₂ but not for ϑ₃, occur in stocks of ϑ_c at a frequency of 10⁻⁷. The mutation from ϑ_c to ϑ_vir is not accompanied by any change of the restriction fragment patterns of DNA.