H. Cerutti et al., INHIBITION OF CHLOROPLAST DNA RECOMBINATION AND REPAIR BY DOMINANT-NEGATIVE MUTANTS OF ESCHERICHIA-COLI RECA, Molecular and cellular biology, 15(6), 1995, pp. 3003-3011
The occurrence of homologous DNA recombination in chloroplasts is well
documented, but little is known about the molecular mechanisms involv
ed or their biological significance. The endosymbiotic origin of plast
ids and the recent finding of an Arabidopsis nuclear gene, encoding a
chloroplast-localized protein homologous to Escherichia coli RecA, sug
gest that the plastid recombination system is related to its eubacteri
al counterpart. Therefore, we examined whether dominant negative mutan
ts of the E. coli RecA protein can interfere with the activity of thei
r putative homolog in the chloroplast of the unicellular green alga Ch
lamydomonas reinhardtii. Transformants expressing these mutant RecA pr
oteins showed reduced survival rates when exposed to DNA-damaging agen
ts, deficient repair of chloroplast DNA, and diminished plastid DNA re
combination. These results strongly support the existence of a RecA-me
diated recombination system in chloroplasts. We also found that the wi
ld-type E. coli RecA protein enhances the frequency of plastid DNA rec
ombination over 15-fold, although it has no effect on DNA repair or ce
ll survival. Thus, chloroplast DNA recombination appears to be limited
by the availability of enzymes involved in strand exchange rather tha
n by the level of initiating DNA substrates. Our observations suggest
that a primary biological role of the recombination system in plastids
is in the repair of their DNA, most likely needed to cope with damage
due to photooxidation and other environmental stresses. This hypothes
is could explain the evolutionary conservation of DNA recombination in
chloroplasts despite the predominantly uniparental inheritance of the
ir genomes.