Effects of low dose actinomycin D treatment in vivo on the biosynthesis of ribosomal proteins in rat liver.

The long-term effects (up to 12 h) of low dose in vivo actinomycin D treatment, which selectively inhibits rRNA synthesis, on the activity of rat liver for the synthesis of ribosomal proteins relative to that for the synthesis of total protein were investigated. The effects of actinomycin D treatment in vivo and in vitro on the template activity of poly(A)-containing mRNA of rat liver for ribosomal proteins were examined by using a wheat germ cell-free system. The following results were obtained. 1. The activity of rat liver for synthesizing total protein observed in vivo and in vitro was inhibited by actinomycin D treatment even at a small dose. 2. A double-labeling technique using [3H] and [14C]leucine in vivo showed that the rate of synthesis of the ribosomal protein fraction relative to that of total protein in actinomycin-treated rat liver (6 + 6 h) was 1.45 times higher than that in the control rat. 3. By using a wheat germ cell-free system, it was shown that the template activity of poly(A)-containing mRNA for the synthesis of total protein was increased slightly by actinomycin D treatment in vivo. Furthermore, the template activity for the ribosomal protein fraction relative to that for total protein was increased. This increase was observed in most of the ribosomal proteins separated on two-dimensional acrylamide gel electrophoresis, although the extents of increase were different among individual ribosomal proteins examined. On the other hand, the selective increase of the template activity for the ribosomal protein fraction was not observed when poly(A)-containing mRNA was incubated with actinomycin D in vitro, although the template activity for total protein was increased slightly.