Analysis of temperature-sensitive mutant ts 187 of Saccharomyces cerevisiae altered in a component required for the initiation of protein synthesis.

Postpolysomal extracts have been prepared from wild type haploid Saccharomyces cerevisiae cells (wt A364A) and from a temperature-sensitive mutant strain (ts 187, gene prt 1). The extracts, prepared via spheroplasts and depleted of endogenous mRNA with nuclease, translate exogenous natural mRNA and polyuridylic acid. The activity of wt A364A with respect to translation of yeast mRNA, poly(U)-dependent synthesis of polyphenylalanine which measures elongation components, reactions involved in the initiation of protein synthesis, and termination and release of polypeptides, is not significantly affected when spheroplasts are incubated at 39 degrees C for relatively short periods of time, prior to the preparation of the cell-free system. With extracts obtained from ts 187 cells, preincubation of spheroplasts at 39 degrees C prior to the preparations of the cell-free system markedly decreases the ability to translate natural mRNA but not poly(U). Compared to extracts from unheated spheroplasts, the following activities in ts 187 extracts from spheroplasts preincubated at 39 degrees C are not significantly affected: activation of methionine and methionylation of tRNAMet; formation of (eukaryotic initiation factor 2.Met-tRNAf.GTP] ternary complex; binding of mRNA to 40S preinitiation intermediate containing Met-tRNAf, and joining of 60S subunits to form the 80S initiation complex; elongation factor 1- and elongation factor 2-dependent elongations reactions; and termination and release of completed polypeptide chains. However, the interaction between the [eukaryotic initiation factor 2.Met-tRNAf.GTP] ternary complex and 40 S subunits, to form the 40 S preinitiation complex, is drastically inhibited by treatment of the spheroplasts at 39 degrees C.