Virus transcription

[Lyle Najita]

>I have though about this and have often wondered (as most
picornavirologist have) just what is the >role of VpG at the 5' end of the
positive strand genome?  If it is involved in priming, why is it on the >5'
end of the positive strand.  Generally a priming protein (if I'm correct)
is associated with the >strand that is read during transcription (in this
case the negative strand).  Is this a different >mechanism and VpG is
transfered during transcription initiation?  Why have 100 nts if secondary
>structure if it is not important in positive strand synthesis?   
>Is VpG involved in translation and has just been ignored for no particular
reason?  
 
The way I remember the lit. VPg (sorry for spelling this wrong on previous
post) is also found on negative-strand RNA. Flanegan and co-workers
hypothesized that the mechanism whereby VPg is added to the negative-strand
RNA was by a trans-esterification reaction resulting in VPg added to the
poly-U stretch which had previously been added by host factor. They had
published a paper demonstrating the addition of chemically syntheized VPg
to "replicative intermediate RNA" which was a portion of polio RNA with a
hairpin where the 3' end would be to resemble poly-A-poly-U, the
presumptive host factor product. As for VPg being present on the
postive-strand, if it is true that positive-strand RNA is primed by
VPgpUpU, then it would make sense that it is there. VPg is also quite
possibly required for packaging of the viral genome. VPg is released before
translation of the viral RNA. Even if VPgpUpU is the primer for
positive-strand RNA synthesis, this wouldn't negate the contribution of any
secondary structure. The secondary structure is probably necessary for
recognition, two U's don't allow for much stabilized base-pairing. Also
keep in mind that there must be some mecanism to distinguish
positive-strand RNA to be unlinked from VPg for translation from VPg-linked
RNA for packaging. 
 
L