Polynucleotide Kinase (PNK) is used to add a phosphate to the 5' end of a DNA
fragment. We phosphorylate oligos for cassette mutagenesis and we
phosphorylate primers for toe printing experiments (basically a fancy primer
extension reaction we use to map the positions of ribosomes on an mRNA).
The best approach is to follow the directions provided by the
manufacturer. They usually provide buffer also. But if it is not
compatible with the next reaction you are planning this buffer works well also.