Significance of rare mRNA sequences in liver

From mRNA-DNA hybridization studies it is known that eukaryotic mRNAs occur in several abundance classes. One such study with mammalian liver mRNA indicates that in this tissue the most complex abundance class consists of messenger RNAs present in 5–40 copies per cell. The other abundance classes are less complex and consist of mRNA sequences present an average of 250 and 7200 times per cell. In this paper we summarize approximate calculations of the number of mRNA molecules needed to sustain the steady-state quantities of 40 rodent liver proteins. Data were obtained from the literature regarding subunit molecular weights, degradation rate constants, and the concentration of each of these proteins in liver. The sample of proteins was chosen simply on the basis of the availability of the relevant data in the literature. For 13 of these proteins more direct estimates of mRNA frequency per cell could also be derived from measurements of the fraction of total protein synthesis accounted for by the protein in question. In all cases these estimates agreed within a factor of 2 to 3 with the values calculated from protein concentration and turnover rates. Several proteins known to be expressed in a histospecific way in liver were found to require steady-state mRNA concentrations in the range of only 1–30 copies per cell. This suggests that at least some of the mRNAs in the lowest abundance class are present as the result of the specific regulation of structural gene transcription