Internal promoters of tRNA genes In many eukaryotic tRNA genes, where are the promoter elements located relative to the transcription start site?

Introduction / Context:
tRNA genes have unique promoter architecture compared with typical protein-coding genes. Understanding this is key to appreciating RNA polymerase III transcription mechanisms.

Given Data / Assumptions:

• tRNA transcription is primarily carried out by RNA polymerase III in eukaryotes.
• Internal promoter elements (A box and B box) reside within the transcribed sequence of tRNA genes.

Concept / Approach:
Unlike many RNA polymerase II promoters, pol III promoters for tRNAs are internal. Transcription factors (TFIIIC, TFIIIB) recognize A and B boxes located downstream of the start site, leading to accurate initiation upstream of these boxes.

Step-by-Step Solution:
Recall that tRNA promoters are internal to the gene.Identify the correct positional description: downstream (within the transcribed region).Select the matching option.

Verification / Alternative check:
Deletion mapping experiments show loss of transcription when A/B boxes inside the tRNA gene are removed.

Why Other Options Are Wrong:

• Upstream only: Typical for pol II, not pol III tRNAs.
• Both in all organisms: Overgeneralization; internal promoters are characteristic for eukaryotic tRNA genes.
• Intergenic enhancers: Not the core promoter elements for tRNAs.

Common Pitfalls:
Projecting pol II promoter logic onto pol III-transcribed genes.

Final Answer:
Downstream from the start site (within the transcribed region).