Metabolically induced shifts in the translational efficiency of yeast spheroplast polysomes

Abstract

This paper presents a study into the in cico translational efficiency of yeast polysomes as their level passes through a cycle of buildup, constancy, and subsequent decline during the incubation of freshly protoplasted cells in growth medium. The in cico rate of protein synthesis was determined from amino acid incorporation data as corrected by two independent methods for the specific radioactivity of a precursor, ciz., the cellular pool of free amino acids and nascent protein. During the initial phase of polysome buildup the translational efficiency is at first very high but declines to a lower value before the maximum level of cellular poly-C ytoplasmic levels of mRNA in eucaryotic cells are generally estimated by methods which are restricted to the assay of bulk mRNA or, at best, to relatively large heterogeneous populations of messengers. With certain restrictions, however, it is possible to experimentally determine the relative changes in the cellular amounts of a given mRNA species from the kinetics of synthesis of the respective protein. This indirect method is based on the assumption that at any given time the rate of synthesis of a protein (dP/dr) is proportional to the amount of its functional mRNA ( M ) , as expressed by the equationwhere k,, is the rate constant for protein synthesis. Thus, if further mRNA synthesis is inhibited, the remaining population of mRNA molecules (Mo) will decay with presumably first-order kinetics and in the process sustain the synthesis of a limited amount of protein (P) which can be computed from the integrated form of eq 1, i.e.where kdm is the rate constant for mRNA decay and t the time elapsed after the inhibition of mRNA synthesis. As t approaches effective infinity the amount of mRNA at time zero can be computed from the reduced form of eq 2, i.e.

Inhibitors of Ribonucleic Acid Synthesis in Saccharomyces cerevisiae : Decay Rate of Messenger Ribonucleic Acid

“…4). It should be noted that no significant protein turnover in the absence of protein synthesis has been observed (35).…”