Strand asymmetry of +1 frameshift mutagenesis at a homopolymeric run by DNA polymerase III holoenzyme of Escherichia coli

We have recently shown that single-base frameshifts were predominant among mutations induced within the rpsL target sequence upon oriC plasmid DNA rep lication in vitro. We found that the occurrence of +1 frameshifts at a run of 6 residues of dA/dT could be increased proportionally by increasing the concentration of dATP present in the in vitro replication. Using single-str anded circular DNA containing either the coding sequence of the rpsL gene o r its complementary sequence, the +1 frameshift mutagenesis by DNA polymera se III holoenzyme of Escherichia coil was extensively examined. A(6) --> A( 7) frameshifts occurred 30 to 90 times more frequently during DNA synthesis with the noncoding sequence (dT tract) template than with the coding seque nce (dA tract). Excess dATP enhanced the occurrence of +1 frameshifts durin g DNA synthesis with the dT tract template, but no other dNTPs showed such an effect. In the presence of 0.1 mM dATP, the A(6) --> A(7) mutagenesis wi th the dT tract template was not inhibited by 1.5 mM dCTP, which is complem entary to the residue immediately upstream of the dT tract. These results s trongly suggested that the A(6) --> A(7) frameshift mutagenesis possesses a n asymmetric strand nature and that slippage errors leading to the +1 frame shift are made during chain elongation within the tract rather than by misi ncorporation of nucleotides opposite residues next to the tract.