homologous recombination: why not?

[Michael Lichten]

Dave Rose writes:
>Does anyone know why it is exactly that yeast has homologous
>recombination while most other systems don't (at least not very
>efficiently)? 

No.  Not exactly.  But the real question is why yeast doesn't repair 
nonhomologously (see below).  

>i.e. is there some selective advantage to being able to do
>this or is it a biproduct of a repair pathway that doesn't exist in most
>organisms? Etc.

Don't assume that most organisms don't do homologous recombination
--they do.  It's just that in most cases, a nonhomologous system prevails 
with exogenously added DNA.  There are exceptions--for example, avian pre-B 
cells, which target exogenous DNA with relatively high fidelity (Jean-Marie 
Buerstedde and coworkers).  And it isn't that clear to me that "other 
systems" don't do most of their repair of breaks on chromosomal DNA 
homologously.  Anyone out there know of any data?

Also note that, absent homology, yeast will integrate nonhomologously (Bob 
Scheistl and Tom Petes).

Finally, it should be pointed out that, although S. cerevisiae repairs 
double-strand breaks on its _chromosomes_ very efficiently by homologous 
recombination, it is not at all clear what the efficiency of homologous 
repair of exogenously added DNA is.  It may very well be that other systems 
are just as "efficient" at homologous integration of exogenously added, non
-chromosomal DNA as is yeast, but that yeast is defficient in or turned way 
down for nonhomologous integration.

This doesn't answer your question, but maybe will help you reconsider it.

Michael Lichten