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RYBICKI, ED ED at molbiol.uct.ac.za
Tue Oct 4 09:18:56 EST 1994

> Ed Rybicki makes a comment about evolution of negative strand 
> to the effect that what has evolved must be advantageous because 
it has
> evolved.  The problem with this proposition is that it is 
circular, and
> doesn't explain anything interesting.  Nothing against Ed, lots of
> people have done the same thing.  

...largely because evolution doesn't necessarily do something 
because it confers an advantage, it sometimes happens because it 

> In the case of negative strand viruses, one significant difference 
> positive strand viruses is that the former can express several 
> mRNAs on infection, at different levels, whereas the latter can 
> only one mRNA, and in order to separate different genetic 
functions early 
> in infection they have to go through the business of separating 
> components of the polyprotein.  

Dear, dear...are we forgetting that (1) some (+)-strand viruses are 
multi-component (and therefore CONSIST of multiple mRNAs); and (2) 
quite a few (+)-strand RNA viruses ALSO express subgenomic mRNA(s) 
from (-)-strand replicative internediates?  And that the MINORITY of 
types of (+)-strand RNA virus (hell, class IV!) actually make - and 
have to separate - components of polyprotein(s)?  I can think of 
only picornaviruses and potyviruses which make an exclusive habit of 
this; many plant viruses employ read-through stop codons to make 
more than protein from one ORF AND make subgenomic mRNAs, and I can 
think of one that makes a polyprotein as well (TYMV), as does 
Sindbis virus....  Not as simple as you make out, I am afraid - in 
fact, the more you argue in this way the more the Class V (-RNA) 
viruses look like being DISadvantaged in the evolutionary stakes 
relative to Class IV.  And I don't believe they are....
 | Ed Rybicki, PhD          |         Well, I tip my hat           |
 | (ed at micro.uct.ac.za)     |      To the new constitution         |
 | Dept Microbiology        | Take a bow for the new revolution... |
 | University of Cape Town  |  Then I get on my knees and pray     |
 | Private Bag, Rondebosch  |   We don't get get fooled again...   |
 | 7700, South Africa       |                                      |
 | fax: xx27-21-650 4023    |      - Pete Townshend, 1972          |
 | tel: xx27-21-650 3265    |      (Won't get fooled again)        |

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