Structure of T4 Bacteriophage Size Approximately 200 nm long and 80-100nm wide Head or Capsid Icosahedral Protective covering for the nucleic acid Tail A hollow tube through which the nucleic acid passes during infection. Surrounded by a contractile sheath. Base Plates Tail fibers

Capsid Sheath Baseplate

The T4 bacteriophage and Its Genome T4 is a bacteriophage that infects E. coli bacteria Complete genome sequence is 169–170 kbp long Encodes about 300 gene T4 biology and its genomic sequence provide the best-understood model for modern functional genomics and proteomics Its tail fibres allow attachment to a host cell. The T4’s tail is hollow so that it can pass its nucleic acid to the cell it is infecting during attachment

Map of the T4 Genome early genes genes with related functions late Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Map of the T4 Genome early genes genes with related functions are usually found clustered together late genes Map of T4 genome. Genes of related function tend to be clustered together. Figure 17.7

The T4 Genome T4 genome is linear dsDNA (169–170 kbp) Gene with related functions–such as genes for phage head or tail fiber construction- are usually clustered together T4 genome encodes product needed for its replication, including all the protein subunits and enzymes needed to prepare for synthesis of DNA. some of these enzymes synthesize hydroxymethylcytosine (HMC), a modified nucleotide that replaces cytosine in T4 DNA

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Synthesis of T4 DNA contains hydroxymethyl-cytosine (HMC) instead of cytosine synthesized by two phage encoded enzymes then HMC glucosylated Figure 17.9

Figure 17.8. T4 encodes modes of the enzymes needed to replicate its dsDNA genome

HMC glucosylation HMC residues protects phage DNA from host (E. coli) restriction endonucleases, which would other wise cleave the viral DNA at specific points and destroy it. Endonucleases: are the chemical scissors/chemical weapons/ molecular weapons which cut/split/cleave the DNA molecule into small fragments. Restriction

T4 DNA is terminally redundant Post synthesis events T4 DNA is terminally redundant base sequence repeated at both ends allows for formation of concatamers long strands of DNA consisting of several units linked together

Concatamer: A concatamer is a long continuous DNA molecule that contains multiple copies of the same DNA sequences linked in series. These polymeric molecules are usually copies of an entire genome linked end to end. Concatemers are frequently the result of rolling circle replication (RCA), and may be seen in the late stage of bacterial infection by phages. As an example, if the genes in the phage DNA are arranged ABC, then in a concatemer the genes would be ABCABCABCABC and so on.They are further broken by ribozymes.

Figure 17.10 The formation of concatemers during replication of T4 genome is an important step in phage reproduction. During assembly of the virions, the phage head is filled with DNA cleaved from the conctamer. Because slightly more than one set of T4 gene is packaged in each head, each virion contains a different DNA fragment. However, if each genome was circularized, the sequence, the sequence of gene would be the same.

Assembly of Phage Particles Complex self-assembly process that involves special virus proteins and some host cell factors. Late mRNA directs the synthesis of three kinds of proteins: (1) Phage structural proteins, (2) proteins that help with phage assembly without becoming part of the virion structure, and (3) protein involved in cell lysis and phage release. Late mRNA transcription begins about 9 minutes after T4 DNA injection into E. coli. All the phages proteins required for assembly are synthesized simultaneously and then used in four fairly independent subassembly lines (Fig 17.11)

The baseplate is constructed of 16 gene products, which are assigned numbers (Fig. 17.12). After the base plate is finished, the tail tube is built on it and the sheath is assembled around the tube. The phage prohead is constructed of 10 proteins. A special portal protein is located at the base of the prohead where it connects to the tail.

Figure 17. 11 The Assembly of T4 Bacteriophage Figure 17.11 The Assembly of T4 Bacteriophage. The subassembly lines for the baseplate, tail tube and sheath, tail fibers, and head.

Release of Phage Particles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Release of Phage Particles in T4 - E. coli system, ~150 viral particles are released two proteins are involved in process T4 lysozyme attacks the E. coli cell wall Holin creates holes in the E. coli plasma membrane, enabling T4 lysozyme to move from the cytoplasm to the peptidoglycan.

Figure 17.12 T4 is composed of numerous proteins, most of which are designated with numbers rather than name.

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.