Two-label peak-height encoded DNA sequencing by capillary gel electrophoresis: three examples

We report a modification to the peak-height encoded DNA sequencing technique of Tabor and Richardson. As in the original protocol, the sequencing reaction uses modified T7 polymerase with manganese rather than magnesium to produce very uniform incorporation of each dideoxynucleoside. To improve sequencing accuracy, two fluorescently labeled primers are employed in separate sequencing reactions. As an example, one sequencing reaction uses a FAM-labeled primer with dideoxyadenosine triphosphate and dideoxycytosine triphosphate; the concentrations of ddATP and ddCTP are adjusted to produce a 2:1 variation in the relative intensity of fragments. The second sequencing reaction uses a TAMRA labeled primer with dideoxythymidine triphosphate and dideoxyguanidine triphosphate; the concentrations of ddTTP and ddGTP are adjusted to produce a 2:1 variation in relative intensity of fragments. The pooled reaction products are separated by capillary gel electrophoresis and identified by one of three different detector systems. Use of a 2:1 peak height ratio typically produces a sequencing accuracy of 97.5% for the first 350 bases; a 3:1 peak height ratio improves accuracy to 99.5% for the first 400 bases. For these experiments, capillary electrophoresis is performed at an electric field of 200 V/cm; two to three hours are required to separate sequencing fragments up to 400 nucleotides in length.