Mutator bacteriophage D108 and its DNA: An electron microscopic characterization

Three types of phage particles were observed on CsCl step gradients when D108 was purified from lysates prepared by induction of a prophage. These particle types were identified to be the mature phage, tailless DNA-filled heads, and a form of nucleoprotein aggregates. The nucleoprotein aggregates banded at a density (ρ) of >1.6. DNA molecules isolated from mature phage particles were (38.305 ± 1.226) kilobases (kb) in length. Denaturation and renaturation of D108 DNA resulted in the formation of linear double-stranded molecules with variable-length single-stranded tails at one end. About 30% of the annealed molecules also carried an internal nonhomology, which was shown to be the region called the G-loop in Mu and P1 DNAs. Following the notation used for different regions of denatured, annealed Mu DNA, we measured the lengths of the equivalent D108 DNA regions to be as α-D108 = (32.178 ± 1.370) kb; G-D108 = (3.07 ± 0.382) kb; β-D108 = (2.291 ± 0.306) kb; SE-D108 = 0.966 ± 0.433) kb. Formation of D108; Mu heteroduplexes disclosed the presence of five nonhomologies, two of which were partial. One of the partial heterologies was in the G-loop region. The largest nonhomology, (1.393 ± 0.185) kb in size, was near the c end (immunity region) and probably spans the c and the ner genes of Mu. β-D108 was shown to carry a (0.556 ± 0.097)-kb insertion close to its right end. A short 100-base-pair region appeared to have been conserved at the ends of D108 and Mu. Occasionally, a 50- to 100-base-pair-long unpaired region was also observed at the left end of D108: Mu heteroduplexes. These sequences were presumably of bacterial DNA. Taken together, our results complement and extend our earlier genetic studies which established that D108 was a mutator phage heteroimmune to Mu with a host range different from Mu's.