The SCR1 gene, coding for the 7SL RNA of the signal recognition particle, is the last known class III gene of Saccharomyces cerevisiae that remains to be characterized with respect to its mode of transcription and promoter organization. We show here that SCR1 represents a unique case of a non-tRNA class III gene in which intragenic promoter elements (the TFIIIC-binding Aand B-blocks), corresponding to the D and T⌿C arms of mature tRNAs, have been adapted to a structurally different small RNA without losing their transcriptional function. In fact, despite the presence of an upstream canonical TATA box, SCR1 transcription strictly depends on the presence of functional, albeit quite unusual, A-and B-blocks and requires all the basal components of the RNA polymerase III transcription apparatus, including TFIIIC. Accordingly, TFIIIC was found to protect from DNase I digestion an 80-bp region comprising the A-and B-blocks. B-block inactivation completely compromised TFIIIC binding and transcription capacity in vitro and in vivo. An inactivating mutation in the A-block selectively affected TFIIIC binding to this promoter element but resulted in much more dramatic impairment of in vivo than in vitro transcription. Transcriptional competition and nucleosome disruption experiments showed that this stronger in vivo defect is due to a reduced ability of A-block-mutated SCR1 to compete with other genes for TFIIIC binding and to counteract the assembly of repressive chromatin structures through TFIIIC recruitment. A kinetic analysis further revealed that facilitated RNA polymerase III recycling, far from being restricted to typical small sized class III templates, also takes place on the 522-bp-long SCR1 gene, the longest known class III transcriptional unit.The most represented RNA polymerase III (Pol III) 1 -transcribed genes, those coding for the tRNAs and the 5 S rRNA, have a highly conserved intragenic promoter comprising the binding sites for the general transcription factor TFIIIC (Aand B-blocks) and for the 5 S-specific factor TFIIIA (C-block). This conservation probably reflects the dual function of the above elements as both nucleation sites for transcription complex assembly and key determinants of tRNA and 5 S rRNA structure. Within the same genes, in fact, an extremely high sequence variability is displayed by the structurally unconstrained, vicinal upstream region. This region provides the binding surface for the initiation factor TFIIIB (1) and can modulate the strength of the intragenic promoter (see Refs. 2-4 and references therein). TFIIIB, which in yeast is minimally composed of the TATA-box-binding protein, the TFIIB-related factor BRF (or TFIIIB70), and the Pol III-specific factor BЉ (or TFIIIB90), is generally assembled on tRNA genes in a TFIIICdependent manner (5). One extreme case of 5Ј-flanking sequence effect, however, has recently been documented for some tRNA genes of Saccharomyces cerevisiae, which, due to the presence of a canonical TATA box in their 5Ј-flanking region, are capable of autonomous ...