You can buy biotinylated, fluoresceinated, or amino-modified ddNTPs from
DuPont NEN (Wilmington, DE). You can use the amino groups on the latter
to attach whatever label or hapten you want. Instead of
streptavidin-alkaline phosphatase, I recommend using
anti-biotin-alkaline phosphatase, or horseradish peroxydase, which will
give you better sensitivity. Check the catalogs of Biodesign, Vector,
Zymed and Boehringer. One reference that might be useful: Nucleic Acids
Research, 1994, Vol. 22, 4167-4175. good luck.