Homologous recombination: why?

Thu Sep 30 11:00:42 EST 1993

>>        Does anyone know why it is exactly that yeast has homologous
>>recombination while most other systems don't (at least not very
>>efficiently)? 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.
>>                                                Thanks.
>>                                                Dave Rose.
Yeast is not especially proficient in homologous recombination; rather it
has notably less efficient nonhomologous repair systems than many other
organisms.  Current studies on mammalian recombination show that
gene-targeting can be quite efficient and that the majority of
"nonhomologous" insertions actually show evidence of first having "picked
up" sequences adjacent to the homologous target.  Why these events then get
inserted somewhere else remains mysterious.  Yeast, on the other hand,
DOES show all the kinds of nonhomologus repair one sees in mammalian cells
(end-joining of DNA fragments, deletions around double-strand breaks,
nonhomologous insertions) but these events are only seen when the
homologous repair systems are knocked out or when there aren't any homologous
partner sequences to effect repair by recombination.  
In organisms with more junk DNA it may be acceptable to repair DNA damage
by less fastidious means.  Yeast has very little "junk" so that
non-homoloogus repair events would be far more likely to create mutations
by deletion, etc.  
                                                        Jim Haber

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