Postmitochondrial cytoplasmic extracts, prepared from uninfected NIH/3T3 cells as well as from chronically or exogenously infected with murine leukemia virus (MLV), were found to stimulate the endogenous reaction of purified MLV reverse transcriptase. No such stimulation was observed with the exogenous reaction of this enzyme, using poly(rA)oligo(dT) as an exogenous templateprimer. While the stimulatory capacity of extracts from uninfected and chronically infected cells was comparable, that of the exogenously infected cells was much more powerfull in this respect. The stimulatory activity could be destroyed by trypsin, indicating that it was excerted by a protein. In uninfected and chronically infected cells this protein was found to be of a short functional life time under conditions blocking continuous protein synthesis. However the mRNA coding for this factor was found in these cells to be stable. On the other hand, the increased stimulatory activity, observed in extract of exogenously infected cells, was independent on protein synthesis and therefore was attributed to a protein apparently introduced into the cells by the penetrating virions. Experiments with monospecific antibodies against MLV proteins suggested that p30 is an important accessory for reverse transcriptase activity and that the cytoplasmic stimulatory factor might be also related to p30.
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