Bacteriophage N4

Abstract Bacteriophage N4 is a lytic phage specific for Escherichia coli K-12 strains originally isolated from the sewers of Genoa (80). N4 is unique in: (i) the use of three different DNA-dependent RNA polymerases during its growth cycle, (ii) a virion-encapsidated RNA polymerase (N4 vRNAP) that is injected into the host cell upon infection, (iii) the use of single-stranded DNA binding proteins as transcriptionalactivators, (iv) the presence of 30 extensions at each end of its linear genome, and (v) a lysis-inhibited infection cycle. The N4 virion particle, as visualized by electron microscopy, consists of an icosahedral head 70 nm in diameter connected by a base plate to a small noncontractile tail, and a number of short tail fibers originating from the junction between the head and the tail (figure 21-1A) (82). The virion particle is composed of a single linear double-stranded DNA molecule and, as determined by sodium dodecyl sulfate^ polyacrylamide gel electrophoresis, 10 proteins (25). The major component of the virion is the coat protein, a 48 kDa polypeptide. Also present is a virion-encapsidated phage-coded, DNA-dependent RNA polymerase (vRNAP) that is responsible for transcription of phage early genes (24).