Features of the cell-free translation of a spider fibroin mRNA

The massive production of fibroin by the large ampullate glands of the spider, Nephila clavipes, serves as a model system in which to study the synthesis and control of a large secretory protein. Their tissue-specific product, fibroin, produced during the entire adult life of the female, is approximately 320 kilodaltons, and rich in glycine and alanine. Highly purified fibroin mRNA from the glands has been translated in a rabbit reticulocyte cell-free system with variable supplements. The translational products analyzed by SDS-PAGE display two features, tRNA modulation and discontinuous pauses during elongation. tRNA complements exert their effects both in the translational efficiency and in the size of the peptides generated. The pauses observed during translation generate subsets of smaller discrete peptides, visualized in the gels as ladders of variable relative intensities, appearing exclusively and concomitantly with the fibroin. Definitive linkage between the discrete peptides and fibroin synthesis process has been established by their selective labeling with specific radioactive amino acids.