PURIFICATION AND CHARACTERIZATION OF A DNA HELICASE, DHEL-I, FROM DROSOPHILA-MELANOGASTER EMBRYOS

We have purified a DNA helicase (dhel I) from early Drosophila embryos , dhel I co-purifies with the single-stranded DNA binding protein dRP- A over two purification steps, however, the proteins can be separated by their different native molecular weight, with dhel activity co-sedi menting with a polypeptide of similar to 200 kDa and a sedimentation c oefficient of 8.6 S, The enzyme needs ATP hydrolysis and divalent cati ons for displacement activity, It is very salt sensitive, having a Mg2 + optimum of 0.5 mM and being inhibited by NaCl concentration >10 mM, Dhel I moves 5'-->3' on the DNA strand to which it is bound, Unwinding activity decreases with increasing length of the double-stranded regi on suggesting a distributive mode of action, However, addition of dRP- A to the displacement reaction stimulates the activity on substrates w ith >300 nucleotides double-stranded region suggesting a specific inte raction between these two proteins.