THE EFFICIENCY OF NUCLEAR PROCESSING OF THE TYROSINE AMINOTRANSFERASEMESSENGER-RNA TRANSCRIPT INCREASES AFTER PARTIAL-HEPATECTOMY

Following a two-thirds partial hepatectomy, an approximately fivefold increase in the levels of nuclear and total mRNA for tyrosine aminotra nsferase was observed at 1 h and 1.5 h, respectively, and a return to the levels of the quiescent state, i.e. the levels found in non-operat ed livers from adrenalectomized rats, was established 16 h post-hepate ctomy. The increase in mRNA levels was not accounted for by a comparab le change in the rate of transcription of the gene which, at 0.5 h pos t-hepatectomy, reached a maximum value that amounted to only 1.4-fold the value for quiescent liver. Subsequent changes in the transcription rate largely accounted for the changes in mRNA levels observed later on. Although tyrosine aminotransferase mRNA levels were equal in quies cent and 16-h-regenerating liver, the rate of transcription of the gen e in quiescent liver was threefold higher than the rate in 16-h-regene rating liver. The maintenance of a higher rate of gene transcription i n quiescent liver, as compared to regenerating liver, was shown to dep end on ongoing protein synthesis. The possibility that the high rate-o f gene transcription was due to blockage or pausing during transcript elongation in quiescent liver was excluded. The inference is that the pronounced increase in tyrosine aminotransferase mRNA levels within 1 h of partial hepatectomy is largely due to a rapid increase in the eff iciency of nuclear processing of the primary transcript.