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Proteins taken directly from the Bombyx mori silk gland have been separated and identified as either fibroin or sericin on the basis of their location within the gland and their amino acid composition. Molecular weights of these polypeptides have been determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis, and by agarose-guanidine chromatography. Fibroin consists of approximately equimolar amounts of two large (350,000) polypeptide chains. These may be the products of distinct fibroin alleles present in hybrid silkworm strains. Sericin, on the other hand, is composed of at least the three largest polypeptides (130,000-220,000) present in a mixture of proteins ranging in size from about 20,000 to 220,000.

We have resolved a previously unidentified factor (TFIIID) that is required for in vitro transcription of polymerase III templates. Our ability to resolve factor D from each of the other components of the transcription machinery (polymerase and transcription factors IIIB and IIIC) allowed us to test the capacity of these separated components to form stable complexes with tRNA genes. We find that none of the individual components binds detectably to tRNA genes, but that certain combinations of transcription factors do bind. Our results show that TFIIID is essential for binding and that formation of a full transcription complex can proceed by either of two different pathways.

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