ARCHAEAL TRANSCRIPTION FACTORS AND THEIR ROLE IN TRANSCRIPTION INITIATION

Archaeal RNA polymerases show a weak ability in vitro to bind to promo ter DNA and/or to initiate transcription with low activity independent of upstream regulatory DNA sequences. Active transcription in vitro a nd in vivo, however, depends strictly on a TATA box resembling the TAT A box of eucaryal polII promoters. This TATA box is recognized by a po lypeptide related to eucaryal TATA-binding protein (TBP) I:hat was for merly designated aTFB. Template competition studies showed that this a rchaeal TATA-binding protein (aTBP) is stably sequestered at the promo ter by interaction with the second archaeal transcription factor, aTFA , which is related to eucaryal transcription factor IIB (TRIIB). The a ssociation of archaeal TFIIB (aTFIIB) with the aTBP-promoter complex l eads to template commitment, indicating that aTFIIB recruits archaeal RNA polymerase to the preinitiation complex. These analyses suggest th e following order for assembly of transcription factors on the archaea l promoter: aTBP, aTFIIB, RNA polymerase, and provide evidence for a c ommon molecular mechanism of transcription initiation by eucaryal RNA polymerase II and archaeal RNA polymerases. The sequence of the genes encoding aTBP and aTFIIB (TFB) showed all the characteristics conserve d in their eucaryal counterparts. The degree of sequence similarity be tween archaeal and eucaryal transcription factors is between 27 to 35% for TFIIB and between 36 to 41% for TBP. The findings discussed here indicate that TBP and TFIIB perform analogous functions in Archaea and Eucarya and show that four essential components of archaeal and eucar yal transcriptional machineries, RNA polymerase, TATA box, TBP and TFI IB are homologous.