RecA protein promotes strand exchange with DNA substrates containing isoguanine and 5-methyl isocytosine

The Escherichia coli RecA protein pairs homologous DNA molecules and promot es DNA strand exchange in vitro. We have examined DNA strand exchange betwe en a 70 nucleotide ssDNA fragment and a 40 bp duplex, in which all G and C residues (at 18 positions distributed throughout the 40 bp exchanged region ) were replaced with the nonstandard nucleosides 2'-deoxyisoguanosine (iG) and 2'-deoxy-5-methylisocytidine (MiC), respectively. We demonstrate that t he nonstandard oligonucleotides are substrates for the RecA protein, permit ting DNA strand exchange in vitro at a rate and efficiency comparable to ex change with normal DNA substrates. This observation provides an expanded ex perimental basis for discussions of potential roles for iG and MiC in a gen etic code. Experiments of this type also provide another avenue for explori ng RecA-facilitated DNA pairing mechanisms.