RNA pol, replication fidelity

[Scott A Hoffman]

Question for anyone.   

Recently, I've attended a few seminars on viral pathogenesis.  An 
interesting topic that seemed to crop up from time to time was that a few 
RNA viruses (most notably Ebola and Dengue) show a high replication 
fidelity.  This is interesting since to my understanding RNA viruses tend 
toward high rates of mutation, the idea is that the replicative machinery 
is inefficient.  The question of course is why?  Why, should one virus 
show high rates of mutation and others not.  Is there any evidence that 
the replicative machinery (RNA pol, etc.)  of some viruses are more 
efficient.  Is there something inherent in the secondary structure of 
some viruses that may lead to lowered mutation rates.  To what degree 
does natural selection play a role?  I could envision a situtation in 
which a number of progeny are made (the replicative machinery is just as 
inefficient as other viruses) but the conditions are so stringent for 
survival that only those sequences that show a few base pair changes can 
still be viable.  This however begs certain other questions.  For 
example, as I understand it, Dengue reaches high virus titers.  As it 
turns out, each virus is as viable as the next, and for good reason:  The 
mosquito vector requires high virus titers to become infected.  No 
vector infection - no transmission.  It seems counterintuitive that 
selective pressures play a huge role, if they did then one would expect 
lower virus titers (unless of course, that virus replication continues 
at very high speeds).  Well, I feel as if I'm beginning to babble. So 
I'll leave the question open.   Does anyone have any ideas, thoughts or
 information?  Do I even have all the facts straight?    Thanks.

Scott Hoffman
Graduate Student
schoffma at badlands.nodak.edu