The effect of calcium on in vitro polyphenylalanine synthesis by rice ribosomes

Abstract

1. In vitro amino acid incorporation by polyribosomes from imbibed rice embryos ( Oryza sativa) was dependent on either Ca 2+ (2 mM) or Mg 2+ (3 mM). Only Mg 2+ (6mM) supported polyphenylalanine synthesis by rice ribosomes with a polyuridylic acid system. However, preincubation of ribosomes, polyuridylic acid, Mg 2+, and phenylalanyl-tRNA resulted in a system capable of subsequent polyphenylalanine synthesis in the presence of either Ca 2+ (2 mM) or additional Mg 2+ (3 mM). Both the polyribosomal and preincubated polyuridylic acid systems gave 50–70 % as much incorporation with Ca 2+ as with Mg 2+. 2. In an effort to determine why Ca 2+ could not support polyphenylalanine synthesis without a preincubation, the effect of both Ca 2+ and Mg 2+ on binding of polyuridylic acid was investigated. Both cations were found to promote functional binding of polyuridylic acid to ribosomes under the preincubation conditions. In addition, both cations supported the binding of an equal amount of [ 14C]phenylalanyl-tRNA to ribosomes. In each case, the bound product was identified as monophenylalanine. Approximately 25 % of [ 14C]phenylalanyl-tRNA bound in the presence of either cation was released by subsequent reaction with puromycin. 3. The data suggest that Ca 2+ may support polypeptide chain elongation but not initiation in this eucaryotic system. The reason Ca 2+ cannot promote polypeptide chain initiation is not clear.