The regenerating rat liver, model for rapid, controlled growth in a mammalian system, was used to investigate molecular events occurring during a transition in growth-rate. This investigation focused on mRNA translation during the early phase of liver regeneration following partial hepatectomy. -- The study included examination of: 1) the translation of polysomal (polyribosomal) mRNA; 2) the possible presence of a polysome-associated factor for stimulating or inhibiting initiation frequency or translation rate; and 3) the synthesis of basic proteins (primarily histones and some ribosomal proteins) during the first 10 h of regeneration. Intact polysomes were isolated, an in vitro translation assay for use with rat liver polysomes (instead of purified mRNA) as template was developed, and a criterion for operationally defining basic proteins was determined. Polysome-associated mRNA, isolated by centrifugation through a discontinuous sucrose gradient, was translated in a wheat germ in vitro translation system; and the translation products were fractionated on a carboxymethyl-cellulose column to quantitate basic proteins. -- It was shown that: 1) the A₂₆₀ polysomes/g liver, an estimate of the quantity of mRNA/g liver, is maximal between 7 and 8 h after partial hepatectomy; 2) an active, polysome-associated factor which influences initiation frequency or translation rate is undetectable; and 3) the peak rate of basic protein systhesis does not occur within the first 10 h postoperatively and thus does not coincide with the peak of polysome content. The peak rate of basic protein synthesis appeared to be much less than that reported in uncontrolled, rapidly growing Ehrlich ascites tumor cells. The results are compatible with the findings of others concerning a) DNA transcription; b) mRNA synthesis, transport, and translation; c) ribosomal protein and histone synthesis early during rat liver regeneration; and d) the absence of a detectable, endogenous, initiation frequency or translation rate-stimulating or inhibiting factor. -- A review of the literature on DNA template activity and on ribosomal proteins and histones and their synthesis following partial hepatectomy in the rat is also included
