Extra-Transcriptional Effects of Chromatin Bound RNA Polymerase III Transcription Complexes

Transcription by RNA polymerase III (Pol III) requires sequential assembly of Pol III-specific transcription factors. At the tRNA gene, the TFIIIC complex recognizes and specifically binds at intragenic promoter elements A-box and B-box and aids the assembly of TFIIIB to upstream of the transcriptional start site. Upon binding, Pol III is recruited near start sites and transcription of tRNA genes is initiated. Apart from transcription of a gene, these bound Pol III complexes influence transcription, chromatin state and genome organization of neighboring RNA polymerase II (Pol II)-transcribed genes. Such effects are known as extra-transcriptional effects of Pol III complex. Our study provides evidence of a unique “extra-transcriptional” activity of assembled Pol III transcription complexes at the tRNA gene that blocks progression of intergenic RNA polymerase II transcription. We demonstrated that the Pol III transcription complex bound to the tRNA gene upstream of the Saccharomyces cerevisiae ATG31 gene protects the ATG31 promoter against readthrough transcriptional interference from the upstream non-coding intergenic SUT467. The protection is predominately mediated by binding of the TFIIIB complex. Failure to block this readthrough resulted in compromised ATG31 translation. Given the recent discovery of widespread pervasive transcription in yeast, protection of neighboring genes from intergenic transcriptional interference may be a key extra-transcriptional function of assembled RNA polymerase III complexes. Our data from RNA-seq analysis demonstrated genome-wide effects of DNA bound Pol III complexes on neighboring chromosomal loci, by comparing expression profiles from tfc6 under-expressing mutants and wild-type S. cerevisiae strains. Reduced TFIIIC occupancy in mutant strains altered Pol II derived transcripts and displayed 5’ or 3’ extension of protein-coding genes, readthrough from non-coding transcripts and increase in the transcription of genes near the potential TFIIIC binding sites, including tRNA genes and putative ETC sites. Interestingly, not all genes in the vicinity of TFIIIC binding sites were transcriptionally mis-regulated, suggesting variable strength of influence on Pol II transcripts by TFIIIC bound sites. Finally, as observed in SUT467-ATG31 readthrough, we anticipated translation defects in 5’ or 3’ extended transcripts in mutants. Overall these genome-wide results suggest much complex regulatory role of Pol III transcription factors bound sites than previously anticipated