Changes in the polyadenylated messenger RNA population during differentiation of Vicia faba root cells

Using cDNA probes, the polysomal polyadenylated RNA populations of Vicia faba meristematic, elongating, and mature root cells were analysed and compared, with respect to complexity, abundance distribution, and sequence representation. These cells express 12 300, 15 800, and 15 200 sequences, respectively, of an average size of 1300 nucleotides, distributed in three frequency classes. Transition from the meristematic to the elongating stage is coordinated with the disappearance of 25-30% of the abundant RNA species (2000 copies per cell) and with the appearance of new transcripts corresponding to 3000-3500 genes expressed at an intermediate level (50-60 copies per cell) and belonging to the rare class (about 4 copies per cell). These new transcripts represent 12.5% of the mass of the mRNA during the elongating stage and are quantitatively modulated during the transition to the mature stage. Thus 80 to 85% of the polysomal polyadenylated RNA species expressed in elongating or mature cells are common to the three cell types.