Unit Gene Expression and Replication in Small DNA Viruses Microviridae Small DNA Viruses of Animals (Papovaviruses)
What do the small DNA viruses have to tell us about viral approaches to gene expression and replication? What are their issues, problems and strategies? Are there common themes?
Microviridae-PhiX174 Historically important virus because… first viral genome to be sequenced (1977) + sense ss DNA circular 5386 nucleotides-very small genome genome infectious by itself model for fundamentals of DNA replication
PhiX174 –among smallest DNA viruses T=1 Capsid Ss + DNA circle Converted to ds immediately upon entry = “RF” 11 gene products identified
Attachment and Penetration of PhiX174 Attaches via vertex & pilot protein H binds to a specific lipopolysaccharide in outer cell membrane of its host, E. coli or S. typhimurium The pilot protein leads the circular genome into the cell by an energy-dependent mechanism The genome attaches to the inner bacterial membrane in preparation for DNA replication and RF formation. Gram Negative Cell Wall
11 Gene Products A: DNA replication (513) A*: inhibit host replication????? (341) B: capsid assembly protein (internal) (120) C: regulation of replication (86) D: capsid assembly protein (external) (152) E: lysis (91) F: major capsid protein (427) G: spike protein (175) H: pilot protein (328) J: DNA binding protein(capsid assembly) (38) K: Burst size regulation???? (56)
Genome is very small for 11 gene products Genome sequencing reveals answer Seven genes overlap at least one other gene Gene for A* overlaps 3 other genes Frameshift or use of alternate AUG codon Non-capsid genes involved
Rolling Circle Replication-”sigma” The RF is made by host enzymes (-) strand template for (+) Multiple copies of (+) made in a chain Single genomes cut out (+) is nicked by A protein
Small genome = Dependence on Host RF is produced by action of host DdDp enzymes Progeny genomes synthesized by rolling circle mechanism (host and viral protein A) Transcription by host DdRp enzyme