LECTURE 15: The Sinsheimer Virus: ØX174

1. General Characteristics
   1. Virionic Components
      • - whole virion = 6.2x10⁶ daltons
      • - DNA = 1.7x10⁶ daltons; (G+C)÷(A+T) = 0.74
      • - capsomers = avg 2.5x10⁴ daltons

   2. Life Cycle
      • How it gets into the bacterial cell

        

      • L = 18 minutes
      • B = 100 (but no burst; instead it leaks out)
      • Cell division is not interrupted!
      • Attachment to CORE of LPS

   3. More about the DNA
      1. It is highly resistant to exo-nucleases

      2. It shows little hyperchromic effect

      3. G not equal C, A not equal T !!!

      4. It is highly susceptible to isotopic decay.

      5. DNA/RNA hybridization experiments.

      6. Sequencing: (1969: Fred Sanger, Clyde Hutchison III, et al)

2. Revisited
   1. Pilot Protein - infection

   2. The Genetic Material

      1. GENETIC MAP OF ØX174

         

         Gene	Location	Function of Gene Product	Daltons
         A	3980-135	RF-DNA replication	65,000
         A*	5387-135	Repress Host DNA synthesis
         B	5077-50	Capsid assembly	25,000
         C	135-390	Measures unit of ssDNA	7,500
         D	390-850	Capsid assembly	14,500
         E	570-840	Lysis	17,000
         F	1000-2280	Facet capsomer	49,000
         G	2400-2920	Apex capsomer	21,000
         H	2930-3915	Spike capsomer
         J	850-960	Core protein and packing	8,500
         K	50-220	Unknown
         O		Clockwise Transcription Origins
         Arrows		Replication Origins (In A: rolling circle origin; In B: Complementary origin leading to RF-DNA)

      2. "+" DNA (from the virion) has its "-" strand synthesized (in the cell), which is then transcribed to m-RNA

         

      3. How overlapping genes can operate:
         1. Transcriptase looks for any "AUG" start codons:

            

         2. It begins reading there and in triplet reading frames from thereafter until it reaches one of the "stop" codons.

            

            Note that only the top reading frame setup will lead to the "stop" codon. The other reading frames read past the "stop" to, presumably, their own "stop's".

   3. Packaging

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