It is th Release of Phage Particles Many their host cells a end of the intracellula after about 22 minutes a and appt are re ased. an endolysin that attacks the cell wall 17.3 p Single-Stranded IIS DNA The phase DNA e of the +DNA genome.The phage is released by host lysis through quite dif ired for phage and TheaeDNA beo man respects from 174 and other ssDNA phages The fdp length(figure 17.1).The circular sDNA lics in attaching to the tip of the pilus;th aid c ase of the pro cap where it connects to the tail.The portal pro the the head.Tail fibe rsattach to the baseplate after the head and tail ually released by a secretory process.Filamentous phage coatPrescott−Harley−Klein: Microbiology, Fifth Edition VI. The Viruses 17. The Viruses: Bacteriophages © The McGraw−Hill Companies, 2002 about 9 minutes after T4 DNA injection into E. coli. All the proteins required for phage assembly are synthesized simultaneously and then used in four fairly independent subassembly lines (figure 17.10). The baseplate is constructed of 15 gene products. After the baseplate is finished, the tail tube is built on it and the sheath is as￾sembled around the tube. The phage prohead or procapsid is con￾structed separately of more than 10 proteins and then spontaneously combines with the tail assembly. The procapsid is assembled with the aid of scaffolding proteins that are degraded or removed after construction is completed. A special portal protein is located at the base of the procapsid where it connects to the tail. The portal pro￾tein is part of the DNA translocating vertex, a structure that helps initiate head assembly and aids in DNA movement into and out of the head. Tail fibers attach to the baseplate after the head and tail have come together. Although many of these steps occur sponta￾neously, some require special virus proteins or host cell factors. 388 Chapter 17 The Viruses: Bacteriophages Head proteins Tail fiber proteins Baseplate proteins Baseplate Tube Tube and sheath Collar Whiskers and neck Mature head with DNA Prohead DNA Prohead Tail fibers Figure 17.10 The Assembly of T4 Bacteriophage. Note the subassembly lines for the baseplate, tail tube and sheath, tail fibers, and head. DNA packaging within the T4 head is still a somewhat mys￾terious process. In some way the DNA is drawn into the com￾pleted shell so efficiently that about 500 m of DNA are packed into a cavity less than 0.1 m across! It is thought that a long DNA concatemer enters the procapsid in an ATP-dependent process until it is packed full and contains about 2% more DNA than is needed for the full T4 genome. The concatemer is then cut, and T4 assembly is finished. The first complete T4 particles ap￾pear in E. coli at 37°C about 15 minutes after infection. Release of Phage Particles Many phages lyse their host cells at the end of the intracellular phase. The lysis of E. coli takes place after about 22 minutes at 37°C, and approximately 300 T4 particles are released. Several T4 genes are involved in this process. One directs the synthesis of an endolysin that attacks the cell wall peptidoglycan. Another phage protein called a holin produces a plasma membrane lesion that stops respiration and allows the endolysin to attack the pep￾tidoglycan. Presumably it forms holes in the membrane. 17.3 Reproduction of Single-Stranded DNA Phages Thus far, only double-stranded DNA phage reproduction has been discussed, with the lytic phage T4 as an example. The reproduc￾tion of single-stranded DNA phages now will be briefly reviewed. The phage of X174, family Microviridae, is a small ssDNA phage using E. coli as its host. Its DNA base sequence is the same as that of the viral mRNA (except that thymine is substituted for uracil) and is therefore positive; the genome contains overlapping genes (see figure 11.20b). The phage DNA must be converted to a double-stranded form before either replication or transcription can occur. When X174 DNA enters the host, it is immediately copied by the bacterial DNA polymerase to form a double-stranded DNA, the replicative form or RF (figure 17.11). The replicative form then directs the synthesis of more RF copies, mRNA, and copies of the DNA genome. The phage is released by host lysis through a different mechanism than used by the T4 phage. The filamentous ssDNA bacteriophages behave quite dif￾ferently in many respects from X174 and other ssDNA phages. The fd phage, family Inoviridae, is one of the best stud￾ied and is shaped like a long fiber about 6 nm in diameter by 900 to 1,900 nm in length (figure 17.1). The circular ssDNA lies in the center of the filament and is surrounded by a tube made of a small coat protein organized in a helical arrangement. The virus infects male E. coli cells by attaching to the tip of the pilus; the DNA enters the host along or possibly through the pilus with the aid of a special adsorption protein. A replicative form is first synthesized and then transcribed. A phage-coded protein then aids in replication of the phage DNA by use of the rolling-circle method (see section 11.3). The filamentous fd phages do not kill their host cell but es￾tablish a symbiotic relationship in which new virions are contin￾ually released by a secretory process. Filamentous phage coat