Initiation of fibroin biosynthesis. I. Isolation of nascent fibroin peptides from the posterior silk gland of Bombyx mori.

1. Peptidyl-tRNA was prepared from the posterior silk gland ribosomes of Bombyx mori on the fourth to fifth days of the fifth instar to explore the initiation process in fibroin biosynthesis. 2. The peptidyl-tRNA was hydrolyzed at an alkaline pH and the resulting nascent peptides were fractionated on Sephadex G-75 and Sephadex G-200 columns into twelve fractions. Each fraction was analyzed for amino acid composition. 3. The nascent peptides of smaller molecular size were rather rich in glutamic and aspartic acids. However, the amino acid composition of the nascent peptides gradually approached that of fibroin as their molecular size increased. 4. A comparison between the nascent peptides of smaller molecular size and the small subunit of fibroin was made in respect to amino acid composition and tryptic peptide map. Considerable similarity between these two proteins was observed. The implications of these results in relation to the initiation process in fibroin biosynthesis are discussed.