The Escherichia coli RecA protein pairs homologous DNA molecules and promotes DNA strand exchange in vitro. We have examined DNA strand exchange between 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 the nonstandard oligonucleotides are substrates for the RecA protein, permitting DNA strand exchange in vitro at a rate and efficiency comparable to exchange with normal DNA substrates. This observation provides an expanded experimental basis for discussions of potential roles for iG and MiC in a genetic code. Experiments of this type also provide another avenue for exploring RecA-facilitated DNA pairing mechanisms.
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