Reagents for modification of protein-nucleic acid complexes:: II.: Site-specific photomodification of mammalian DNA polymerase β complexes with primers extended by dCTP exo-N-substituted arylazido derivatives

Substrate properties of the earlier synthesized and characterized dCTP derivatives bearing in the exo-n-position of cytosine 2-(4-azido-2,3,5,6-tetrafluorobenzoylamino)ethyl (I), 2-(2-nitro-5-azidobenzoylamino)ethyl (II), 2-(4-azido-2,3,5,6-tetrafluorobenzylideneaminooxymethylcarbolamino)ethyl (III), 4-(4-azido-2,3,5,6-tetrafluorobenzylideneaminooxy)buytloxy (IV), or 4-(4-azido-2,3,5,6-tetrafluorobenzylidenedrazinocarbonyl)butylcarbonylamino (V) groups were studied in the primer extension reaction catalyzed by rat DNA polymerase beta. Unlike the earlier results obtained with HIV reverse transcriptase, dCTP derivatives (I)(III) were not recognized by rat DNA polymerase beta as dTTP analogues, and all the five nucleotides were utilized as dCTP analogues. When compared with dCTP, K-m values for the synthesized dCTP derivatives were higher by a factor of 4-20; V-max were 1-2.3 times higher for (I)-(III) and (V) but 20-fold lower for derivative (IV). Site-specific photomodifications of the primer-template-DNA polymerase beta complexes were carried out using photoreactive reagents PRI-PRV obtained in situ by extension of 5'-P-32-labeled primers with dCTP analogues (I)-(V), respectively, when exposed to UV irradiation at 303-313 nm. Reagents PRI and PRIV provided the maximum photocrosslinking of the 5'-P-32-labeled primer to the DNA template (56%) and to the enzyme (20%), respectively. The lowest efficiency of photocrosslinking was observed for PRII (about 1%).