HOMOLOGOUS RECOGNITION PROMOTED BY RECA PROTEIN VIA NON-WATSON-CRICK BONDS BETWEEN IDENTICAL DNA STRANDS

The RecA protein of Escherichia coli forms a nucleoprotein filament th at promotes homologous recognition and subsequent strand exchange betw een a single strand and duplex DNA via a three-stranded intermediate. Recognition of homology within three-stranded nucleoprotein complexes, which is probably central to genetic recombination, is not well under stood as compared with the mutual recognition of complementary single strands by Watson-Crick base pairing. Using oligonucleotides, we exami ned the determinants of homologous recognition within RecA nucleoprote in filaments. Filaments that contained a single strand of DNA recogniz ed homology not only in a complementary oligonucleotide but also in an identical oligonucleotide, whether their respective sugar-phosphate b ackbones were antiparallel or parallel, and a filament that contained duplex DNA showed the same polymorphic versatility in the recognition of homology. Recognition of self by a filament that contains a single strand reveals that RecA filaments can recognize homology via non-Wats on-Crick hydrogen bonds. Recognition of multiple forms of the same seq uence by duplex DNA in the filament shows that it primarily senses bas e-sequence homology, and suggests that recognition can be accomplished prior to the establishment of new Watson-Crick base pairs in heterodu plex products. However, unlike the initial recognition of homology, st rand exchange is stereospecific, requiring the proper antiparallel ori entation of complementary strands.