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Coat protein mediated protection?

RYBICKI, ED ED at micro.uct.ac.za
Fri Apr 29 04:44:33 EST 1994

> 	Does anyone know what the current opinion on the mechanism of
> Coat Protein Mediated Resistance in plants? As far as I understand 

As someone who has just managed to get a PhD student thru a CP and 
antisense CP transgenic thesis project, I think I can say that it 
looks like CP acts at several levels: there is the inhibtion of 
uncoating level, which has definitely been shown to work for TMV; 
there is inhibition of transport from cell to cell (? coats RNA which 
otherwise would complex with movement protein and stops it getting 
thru plasmodesmata?); and then - there is the black box.  

> Also is antisense RNA
> mediated protection having any success yet? 

My student had success with two viruses (sorry, not published yet), 
but lower level resistance than with CP.  Answer is it sometimes 
works, and sometimes not, but in any case probably won't ever be as 
successful as CP, for the simple reason that antisense tech works best 
against RNAs that are made in the nucleus, and most plant virus RNAs 
are not - so the antisense RNA has to get out into the cytoplasm 
(which it may not do that effectively) where it will be considerably 
diluted compared to being in the nucleus, and THEN find its target 
(which may be coated with movement and/or CP, or be complexed in 
vesicles in a replicative complex), and THEN anneal to it.

> Finally (the last question) does anyone have any opinions on the
> proposal by Greene and Allison (Science, 263 March 11th 1994) and 
the subsequent comments by
> Falk & Bruening (same issue), and Hull (on this group) as to the 

Strikes me that the Science paper was a very specialised look at a 
potential problem - and half the experiment was left out.  Where was 
the transgenic plant expressing a 3'-truncated CP mRNA?  If the 
transgene is expressing an RNA which has an origin of replication 
(=replicase binding site at 3'end), I would think the odds of it 
becoming involved in some sort of recombination are markedly higher 
than if it had no such site: after all, Bujarski's group (with BMV) 
and Baltimore's crowd (with poliovirus) showed that it was negative 
strand template choice which was the predominant cause of 
intramolecular recombination in RNA class 4 viruses at any rate 
(ssRNA, plus strand polarity).  If an RNA has no 3' rep site, then 
to my mind it is less likely to be caught up into replicative 
complexes, and less likely to recombine.  So leave the viral sequences 
off the 3' end when you make transgenes for CPs or anything else 
derived from RNA viruses.  I do hope they didn't salami slice the work 
for more publication; our journal club agreed the 3' deleted construct 
was a control which looked like it should have been essential in order 
for the paper to have been published in the first place.

	IMHO, of course....

 | 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     |
 |                             |   We don't get get fooled again...   |
 | Private Bag, Rondebosch     |                                      |
 | 7700, South Africa          |         - Pete Townshend             |
 | fax: xx27-21-650 4023       |      (Won't get fooled again)        |

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