EXTERNAL PROOFREADING OF DNA-REPLICATION ERRORS AND MAMMALIAN AUTONOMOUS 3'-]5'-EXONUCLEASE

Mammalian nuclear DNA polymerases alpha and beta are known to be devoi d of the editing 3' --> 5' exonucleolytic activity. The base substitut ions misinserted by these polymerases could be eliminated with two kin ds of an 'external' proofreading carried out (1) by the 3' --> 5' exon uclease function intrinsic to DNA polymerases delta and epsilon or/and (2) by the autonomous 3' --> 5' exonucleases non-associated covalentl y with DNA polymerases. DNA polymerases delta and epsilon can be separ ated from autonomous 3' --> 5' exonucleases by means of sedimentation. Ultracentrifugation of the nuclear extracts and cytosols from normal and regenerating rat liver as well as from total embryos has shown the bulk of the cellular 3' --> 5' exonucleolytic activity is due to auto nomous nucleases. Moreover, the level of such a specific activity corr elates with the replicative status of the organs from adult animals: s pleen > regenerating liver) normal liver > cardiac muscle > brain, max imum difference being an order of magnitude. In addition, autonomous e xonucleases were shown to be the constituents of the multienzyme forms of DNA polymerases alpha and beta. Hence, antonomous 3' --> 5' exonuc leases seem to be the principal participants in an 'external' proofrea ding.