Direction of reading of the genetic message

Abstract

The assembly of polypeptide chains during protein biosynthe- sis is believed to proceed from the NHz-terminal through the COOH-terminal amino acid (l-4). Hence, the most direct method for ascertaining the direction in which the genetic message is read is to determine the end location o f a given amino acid in polypeptide chains synthesized in a cell-free system under the direction of synthetic polynucleotides having a codon o f specified base sequence at one end o f the chain. Pre- vious experiments (5) were inconclusive because o f (a) presence o f nucleases in the system, (b) insufficient characterization o f the polynucleotide messenger, and (c) difficulty o f performing end group assays because of the insolubility of the phenylalanine peptides formed. All of these obstacles have now been removed through (a) the use o f a system low in nuclease activity con- sisting o f purified Escherichia coli ribosomes and Lactobacillus arabinosus supernatant and (b) the preparation and unequivocal characterization o f short polyadenylic acid messengers with 1 cytidine residue (and therefore an AAC codon) at the 3’.end. Lysine polypeptides are soluble in water and can be readily characterized.