Constitutive and silk gland-specific tRNAAI" genes from silkworms have very different transcriptional properties in vitro. Typically, the constitutive type, which encodes tRNAVa, directs transcription much more efficiently than does the silk gland-specific type, which encodes tRNA,. We think that the inefficiency of the tRNAsAG gene underlies its capacity to be turned off in non-silk gland cells. An economical model is that the tRNASAGa promoter interacts poorly, relative to the tRNAA'a promoter, with one or more components of the basal transcription machinery. As a consequence, the tRNASAsG gene directs the formation of fewer transcription complexes or of complexes with reduced cycling ability. Here we show that the difference in the number of active transcription complexes accounts for the difference in tRNAA3la and tRNASAsG transcription rates. To determine whether a particular component of the silkworm transcription machinery is responsible for reduced complex formation on the tRNASAG gene, we measured competition by templates for defined fractions of this machinery.We find that the tRNAsG gene is greatly impaired, in comparison with the tRNAcA'a gene, in competition for either TFIIIB or RNA polymerase III. Competition for each of these fractions is also strongly influenced by the nature of the 5' flanking sequence, the promoter element responsible for the distinctive transcriptional properties of tRNAAa and tRNAtIa genes. These results suggest that differential interaction with TFIIIB or