FUNCTIONAL INTERCHANGEABILITY OF TFIIIB COMPONENTS FROM YEAST AND HUMAN-CELLS IN-VITRO

In eukaryotes, TFIIIB is required for proper initiation by RNA polymer ase III, In the yeast Saccharomyces cerevisiae a single form of TFIIIB (yTFIIIB) is sufficient for transcription of all pol III genes, where as in extracts derived from human cells two different hTFIIIB complexe s exist which we have previously designated as hTFIIIB-alpha and hTFII IB-beta, Human TFIIIB-alpha is a TBP-free entity and must be complemen ted by TBP for transcription of pol III genes driven by gene external promoters, whereas hTFIIIB-beta is a TBP-TAF complex which governs tra nscription from internal pol III promoters, We show that hTFIIIB-beta cannot be replaced by yeast TFIIIB for transcription of tRNA genes, bu t that the B '' component of yTFIIIB can substitute for hTFIIIB-alpha activity in transcription of the human U6 gene, Moreover, hTFIIIB-alph a can be chromatographically divided into activities which are functio nally related to yTFIIIE and recombinant yB(90)'', suggesting that hTF IIIB-alpha is a human homolog of yeast TFIIIB '', In addition, we show that yeast TBP can only be exchanged against human TBP for in vitro t ranscription of the human and yeast U6 gene but virtually not for that of the yeast tRNA(4)(Sup) gene. This deficiency can be counteracted b y a mutant of human TBP (R231K) which is able to replace yeast TBP for transcription of yeast tRNA genes in vitro.