Direction of synthesis of messenger RNA in cells of Escherichia coli.

Abstract

Temperature reduction to 0[degree] was used to slow the synthesis of mRNA in intact cells of E. coli. This made it possible to label nonuniformly with C14-uridine. Radioactivity in the 3[image]-terminal nucleosides released by alkaline hydrolysis became constant very early, whereas radioactivity in the nucleotides increased linearly. The polarity of mRNA synthesis is therefore 5[image][forward arrow]3[image]. The average assembly rate of a mRNA strand at 0[degree] is about 1/13 residue/sec., or about 1/40 codon/sec. Since the polarities of mRNA synthesis and of protein synthesis are the same, ribo-somes may synthesize protein on strands of nascent mRNA. Some ribosomes may translate mRNA cistrons whose synthesis is already complete. There are about 800-1,1000 strands of nascent mRNA/cell, probably about the num -ber of operons in E. coli. There is too much nascent mRNA to be saturated by the 5,000 active ribosomes, so not all the nascent messages could be translated simultaneously.