ch25_37 - The Viruses; Human Diseases Caused by Viruses...

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The Viruses; Human Diseases Caused by Viruses & Prions
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Steps in T4 Phage Multiplication 1. Adsorption (or attachment) Virus binds to receptors on host cell via spikes or peplomers Host cell receptors include flagella, pili and lipopolysaccharides 2. Penetration by direct injection Virus capsid/tail fibers left outside host
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Steps in T4 Phage Multiplication 3. Viral Synthesis Copy and express viral genes by host machinery and replicate viral genomes Early proteins are transcribed and translated Occurs before viral nucleic acids are replicated Includes proteins needed to direct viral replication, take over host cell machinery, degrade host cell DNA Viral genome is replicated by rolling circle mechanism Late proteins are transcribed and translated Includes proteins of the virus coat and other structural proteins, assembly proteins, and proteins involved in cell lysis
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Steps in T4 Phage Multiplication 4. Maturation Assembly into intact virions Capsids (or proheads) are packed with nucleic acids Tail fibers (if present) assembled and added to capsids of complex phages Proteins for spikes assembled to capsid of naked virions
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Steps in T4 Phage Multiplication 5. Release Virus particles escape from host cell Cell lysis occurs and virions are ready to infect new hosts Phages encode endolysin proteins which aid in peptidoglycan and cell membrane Other phages do not always lyse their host cell for virion release Some filamentous phage are secreted through the host cell’s plasma membrane
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Lytic Phage Release
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Non-Lytic (Secretory) Phage Release Phage coat proteins insert into the bacterial cell membrane Coat forms around the viral DNA during secretion Host cell growth and replication is reduced, but hosts are not lysed
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Steps in T4 Phage Multiplication
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T4 Phage Multiplication Outcome Complete the entire multiplication cycle in about 22 minutes E. coli generation time ~ 20 minutes Produce approximately 300 new T4 particles per host cell
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O Activator (lambda) binding site for P RM
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Phage λ Lysogeny? A Race Between Regulators Depends on the activities of repressors Lambda repressor protein (product of cI gene) Active form is a dimer – one unit binds to DNA and the other unit binds to the first lambda unit Presence inhibits Cro protein synthesis = promotes lysogeny and transcription of integrase occurs Cro repressor protein (product of cro gene) Also a dimer Binds to the lambda repressor gene = promotes lytic cycle Enhances transcription of viral genes involved in lytic reproduction
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Phage λ Lysogeny? A Race Between Regulators Lambda repression can occur Binds to two operators (O L and O R ) = no transcription of most viral genes; only integrase is transcribed At O L : excisionase is (xis) is repressed and the phage must remain intergrated At O R : lytic cycle reproductive cycle genes are repressed as well as cro protein production
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ch25_37 - The Viruses; Human Diseases Caused by Viruses...

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