Sequence complexity of nuclear and polysomal RNA in leaves of the tobacco plant.

The first measurements are reported of the sequence complexity of nuclear and polysomal RNA contained within the cells of a higher plant. Polysomal RNA from tobacco leaves was prepared by a procedure which minimized contamination with nuclear RNA. Hybridization of 3H-cDNA complementary to polysomal poly(A) RNA with an excess of tobacco DNA indicated that greater than 95% of the poly(A) mRNA was transcribed from single-copy sequences. RNA excess hybridization reactions with polysomal poly(A) RNA and 3H-cDNA revealed the presence of three abundance classes in the poly(A) mRNA. The best least-squares solution indicated that these classes comprise 9, 52, and 39% of the poly (A) mRNA and contain sequences present an average of 4500, 340 and 17 times per cell. Hybridization reactions containing an excess of nuclear or total polysomal RNA and 3H-single-copy DNA indicated the complexity of these RNA populations to be 1.19 X 10(8) nucleotides (nuclear) and 3.33 X 10(7) nucleotides (polysomal). Thus only 28% of the nuclear RNA sequence diversity (27,000 average-sized mRNA sequences) is respresented in leaf polysomes. These results suggest that there is a general similarity in the basic transcriptional processes of metaphytan and metazoan cells.