The requirement for the A block promoter element in tRNA gene transcription in vitro depends on the ionic environment.

When yeast cell extracts that faithfully transcribe class III genes are provided with different electrolyte ions, the pattern of transcripts changes. A transcription unit in pBR322, silent with 0.1M potassium chloride, becomes active in the presence of 0.1M potassium acetate. This pseudogene depends on transcription factors B and C and RNA polymerase III like a tRNA gene. The transcribed region contains the only sequence in pBR322 homologous to the modified B block consensus sequence GTTCRDNNC found in normal tRNA genes. The presence of a block A sequence is less evident. When a block A deleted tRNA(GLU) gene was constructed, it behaved similarly: poorly transcribed with 0.1M potassium chloride, well transcribed with 0.1M potassium acetate. In fact, the deletion of the A block promoter element from the tRNA(GLU) gene did not dramatically lower its transcription when tested with potassium acetate, while it had a strong negative effect when tested with potassium chloride. Consequently the requirement for this promoter element is not constant but is a function of the electrolyte composition.