SILKWORM TFIIIB BINDS BOTH CONSTITUTIVE AND SILK GLAND-SPECIFIC TRNA(ALA) PROMOTERS BUT PROTECTS ONLY THE CONSTITUTIVE PROMOTER FROM DNASE-I CLEAVAGE

We have identified a complex between TFIIIB and the upstream promoter of silkworm tRNA(Ala) genes that is detectable by gel retardation and DNase I footprinting, Formation of this complex depends on the integri ty of previously identified upstream promoter elements and on the pres ence of other silkworm transcription factors, either TFIIID or a fract ion that contains both TFIIIC and TFIIID, We have used this complex to compare the interactions of TFIIIB with two kinds of tRNA(Ala) genes whose different in vitro transcription properties are conferred by the upstream segments of their promoters, These are the tRNA(C)(Ala) gene s, which are transcribed constitutively, and the tRNA(SG)(Ala) genes, which are transcribed only in the silk gland, We find that TFIIIB bind s tRNA(SG)(Ala) genes with lower affinity than it binds tRNA(C)(Ala) g enes, In addition, the TFIIIB complexes formed on tRNA(SG)(Ala) genes differ qualitatively from those formed on tRNA(C)(Ala) genes, Both the transcriptional activity of tRNA(SG)(Ala) complexes and the ability o f the complexes to protect upstream DNA from DNase I digestion are red uced.