Studies of chloroplast DNA variations, and several direct experimental observations, indicate the existence of recombination ability in algal and higher plant plastids. However, no studies have been done of the biochemical pathways involved. Using a part of a cyanobacterial recA gene as a probe in Southern blots, we have found homologous sequences in total DNA from Pisum sanvwn and Arabidopsis thaEaa and in a cDNA library from Arabidopsis. A cDNA was cloned and sequenced, and its predicted amino acid sequence Is 60.7% identical to that of the cyanobacterial RecA protein. This finding is consistent with our other results showing both DNA strand transfer activity and the existence of a protein of the predicted molecular mass crossreactive with antibodies to Escherichia coli RecA in the stroma of pea chloroplasts.The observation of chloroplast DNA recombinants in somatic hybrids of higher plants (1), genetic studies of the inheritance of chloroplast markers in several crosses of Chlamydomonas (2-4), the integration of donor DNA by homologous recombination in chloroplasts of transformed Chlamydomonas (5), and extensive comparative analyses of chloroplast genome structure (6-9) indicate that DNA recombination occurs in chloroplasts of both higher plants and green algae. The biochemistry of any recombinational mechanism in chloroplasts is completely unknown, however.It is generally accepted that plastids originated from cyanobacterial progenitors, acquired by an ancestral eukaryotic cell through an endosymbiotic event (10,11). Therefore it seemed probable that any chloroplast recombination system should be related to a eubacterial counterpart. In Escherichia coli and many other prokaryotes, the RecA protein is essential for homologous recombination and for a variety of SOS responses to DNA damage (12)(13)(14)(15)(16)(17)(18)(19)(20)