14. Discontinuous Translation and mRNA Structure of the Coding Region

Mitsuo Zama

Keywords: translation, mRNA struture, silk fibroin

Discontinuous translational elongation of polypeptides observed during the synthesis of proteins, silkworm fibroin, type I collagen, colicin A, chloroplast photosystem II reaction center protein DI , globin and spider fibroin, has been suggested to be attributable to the mRNA secondary structure of the protein-coding region by our preceding studies. We describe here the result of a further analysis on the correlation between discontinuous translation and mRNA structure in silkworm fibroin synthesis.

The products of fibroin being translated exhibit the ladder of electrophoretic bands smaller than fullsize fibroin in the SDS polyacrylamide gets, due to discontinuities or pauses in the translational proc ess. Our detailed analysis of the published data of the electrophoresis of the fibroin heavy chain showed that the total number of the electrophoretic bands in the get is about 60. On the other hand, it is known that most of the fibroin heavy chain gene (15kb) is composed of alternate arrays of crystalline and noncrystalline elements. By an analysis of the nucleotide sequence of the 3' end cDNA alone of the fibroin heavy chain gene of Bombyx mori available in the literature, we estimated that the total number of the alternate arrays repeating over the whole heavy chain gene is about 60. The results indicate that about 60 pauses occur during the translation of the mRNA. Furthermore, the coincidence of the total number of the electrophoretic bands with that of the alternate arrays suggests that one translational pause occurs at a specific site in each of the alter nate arrays in the fibroin mRNA template.