Viruses and temperate bacteriophage

• biology of viruses

  • Non-living entities – hence not in the tree of life
  • There are viruses that infect cells of all of the kingdoms
  • Utilise host proteins upon infection for their propagation
  • Extracellularly exist as inert virions
  • Have genome sizes from 5Kb to 1.2MBp
  • Extracellularly exist as inert virions
• lytic pathway

  viral DNA replicates, the coat proteins are synthesised and the viral particles burst out of the cell during cell lysis 
• lysogenic pathway

  • most of the viral genes are not expressed and the viral genome (prophage), intergrates into the host chromosome
  • as a prophage, the viral DNA is replicated with the host DNA during cell division
• early events in lambda bacteriophage infection

  • infection by lambda phage begins when the virus attaches to a receptor on the bacterial cell wall and injects its DNA into the cell
  • the protein coat remains outside the cell while the phage DNA inside quickly forms into a circle
  • whether the phage takes the lytic or the lysogenic pathway depends on which genes dominate
  • if cl (lambda repressor) dominates, phage will progress into the lysogenic pathway
  • if cro dominates, the phage will progress into the lytic pathway
• lytic pathway process

  1. Transcription, translation of N and cro
  2. Longer transcripts are made with the help of N protein
  3. The stimulation of phage structural gene synthesis by the Q protein 
• 1. Transcription, translation of N and cro

  • lambda DNA is first transcribed by E.coli RNA polymerases, beginning at two key promoter sequences called PL and PR (promoter left and right)
  • two transcription termination sequences cause the polymerases to stop transcription after only a few genes - N and cro - have been copied
  • the N and cro mRNAs are then translated by the E.coli host ribosomes into the N and cro proteins
• 1. Longer transcripts are made with the help of N protein

  • N protein that has recently been translated from the N gene mRNA now acts as a anti-terminator to block the action of the termination sequences
  • RNA polymerases starting at PL and PR transcribe through the termination sequences and create longer transcripts that include all the cll and clll genes
  • ribosomes translate the mRNAs producing cll and clll proteins, along with the previously produced N and cro proteins
• 1. The stimulation of phage structural gene synthesis by the Q protein

  • during the second phase of transcription when the transcripts are not truncated by the first termination sequences, another gene called Q is transcribed and translated
  • this gene lies clockwise around the chromosome a little beyond the cll gene and is transcribed by polymerases starting at PR
  • Q protein is another anti-terminator. it negates the terminating effects of a second set of termination sequences just beyond the Q gene on the DNA
  • N and Q proteins allow transcription from PR to occur continuously around the chromosome resulting in the production of the head, tail and fibre proteins
  • lambda phages then assemble and burst out of the cell, completing the lytic pathway
• lysogenic pathway process

  1. Lambda repressor synthesis from PE
  2. cl blocks synthesis from PR and the lytic pathway genes
• 1. Lambda repressor synthesis from PE

  • lysogenic and lytic pathways are the same up until the production of cll and clll proteins
  • lysogeny begins when the cl gene is transcribed and translated
  • cll protein activates transcription at a promoter site called PE
• 1. cl blocks synthesis from PR and the lytic pathway genes

  • the cl and cro proteins compete with each other for the domination of transcriptional events
  • OR has three binding sites that cl binds to
  • binding of cl to OR and OL turns off gene expression from PR and PL and blocks the lytic pathway
  • cl has a higher affinity for the operator sites than cro and can bind to them at lower expression levels