Question

8. Which of the following describes the initiation of bacterial transcription

A. The small ribosomal subunit binds to the Shine-Dalgarno sequence

B. The sigma factor binds at the -10 location

C. Transcription begins at the +1 site which is always the start codon

D. RNA polymerase disassociates with the sigma factor as it begins adding nucleotides at the 3’ end

Answer 1

Options B, C and D describes the initiation of bacterial transcription.

B. The sigma factor binds at the -10 location - In bacteria, the promoter consists of two short sequences at -10 and -35 positions upstream from the transcription start site. The sigma factor of the holoenzyme of RNA polymerase binds to -10 and -35 promoters to recognize them.

C- Transcription begins at the +1 site which is always the start codon - The transcription start site of prokaryotes is represented as +1 site, where the first 5' mRNA nucleotide is added. +1 site is located downstream to the -35 and and -10 promoters.

D. RNA polymerase disassociates with the sigma factor as it begins adding nucleotides at the 3’ end - The RNA polymearse involved in bacterial transcription is composed of five polypeptide subunits such as two alpha subunits, one beta subunit, one beta prime subunit and an mega subunit. These 5 subunits together forms the core enzyme. But during the promoter scanning, the RNA polymerase enzyme is in the holoenzyme state where an additional subunit called sigma factor is also included. The holoenzyme of RNA polymerase has low affinity since its main function is to search for the promoter. Once the promoter is recognized, the RNA polymerase enzyme dissociates from the sigma factor. That is, the holoenzyme is converted to core enzyme, which has a high affinity.

Option A - the small ribosomal subunit binds to the Shine-Dalgarno sequence - is a step that describes bacterial translation, not transcription.