Practice question

T4 polynucleotide kinase possesses 5'-kinase activity transferring the gamma-phosphate of ATP to 5'-hydroxyl ends of DNA or RNA, plus 3'-phosphatase activity. Biochemical studies demonstrate that ammonium ions act as potent inhibitors, even at concentrations as low as a few millimolar, by interfering with active site conformation. Because many DNA purification protocols use ammonium acetate for precipitation, residual ammonium can block phosphorylation and subsequent ligation. Therefore, sodium acetate precipitation followed by ethanol washing is preferred before kinase treatment to ensure efficient 5'-end labeling and vector phosphorylation.