The RNA-dependent DNA-polymerase activity was studied in the postmicrosomal fraction and in the microsomal sediment of the liver of the newborn and adult Wistar rats. In the microsomal sediment of 4-6 day old rats the RNA-dependent DNA-polymerase activity was approximately by one order of magnitude higher than in that of 2 week old and adult rats. In the postmicrosomal fraction of 3 day old rats the RNA-dependent DNA-polymerase activity was also higher, but only by 30-35%, than in that of animals of the other age groups. Particles with density of 1.17 g/ml were found in the microsomal sediment of all studied animals. The characteristic morphology, sensitivity of the particle RNA-dependent DNA-polymerase activity to RNAse and the presence of reverse transcriptase allow for these particles to be referred to as retroviruses. A suggestion is put forward that the high intensity of reverse transcription during the early postnatal period can be due to still continuing processes of cell differentiation and enzymatic imprinting.
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