A method for the study of membrane-binding of ribosomes and the rate of synthesis of protein on free and bound ribosomes has been examined critically. 'Brij 58' has been found to be more suitable for detaching ribosomes from membranes than 'Triton X-100' or sodium deoxycholate since it does not alter the structure or biological activity of the ribosomes. Triton and deoxycholate remove amino acid- and ribonucleotide-containing material from ribosomes, thus decreasing their activity during protein synthesis in a cell free system. 2. The rate of protein synthesis on membrane-bound and free ribosomes, the total amount of ribosomes, and the percentage distribution of free and membrane-bound ribosomes were determined in a culture of synchronously growing yeast cells. The results show that in all probability protein synthesis in yeast cells occurs mainly on membrane-bound ribosomes. Free ribosomes are considered here to be a ribosome pool which allows the cell to increase its rate of protein synthesis rapidly. Membrane-bound and free ribosomes are viewed as different functional states, one being a state of protein synthesis and the other a latent state.
