Gene expression and the diversity of polysomal messenger RNA sequences in regenerating liver.

We have determined the percentage of the genome transcribed into nuclear RNA and the sequence complexity and diversity of polysomal mRNA in the livers of sham-operated and partially hepatectomized rats. Both in livers of sham-operated rats and in regenerating livers at the time of maximal DNA synthesis, approximately 12% of the single copy genome is expressed. The complexity of polysomal polyadenylated RNA from livers of sham-operated and partially hepatectomized rats was determined by kinetic analysis of hybridization curves of polysomal mRNA and [3H]cDNA. The sequence complexity of polysomal polyadenylated RNA in 12-, 24-, and 72-h regenerating liver ranged from 10,000 to 20,000 sequences and did not differ from that of nongrowing liver, with the exception of the 72-h value. However, heterologous hybridizations between mRNA from regenerating livers and C3H]cDNA transcribed from polysomal mRNA from livers of sham-operated rats as well as the converse reactions, did not provide evidence for the existence of polysomal mRNA sequences which are specific for either regenerating or nongrowing livers. These studies suggest that changes in frequency of various sequences, rather than qualitative differences, constitute the major alteration in polysomal mRNA populations during liver regeneration. The same conclusion applies to the regenerating liver of weanling rats which show a heightened and more synchronized regenerative response than adult rats.