Transcription in Bacteria

Transcription in bacteria is less complex than transcription in eukaryotes.

Bacteria have promoter regions, a region on the DNA that tells RNA polymerases where to start. RNA polymerases are weakly attached to the DNA and then zip off down the template strand to transcribe. The double helix is opened up immediately, exposing the DNA’s nucleotides and allowing the RNA polymerase to then complement nucleotides accordingly.

RNA polymerases have the ability to transcribe left-right or right-left, but must travel in the direction of 5′ to 3′ (which means that the RNA polymerase travels down a DNA template strand of 3′ to 5′).

RNA polymerase’s mRNA strand [[5‘ to 3‘]]

DNA Template Strand                    [[3‘ to 5‘]]

DNA has two strands, so which strand is the template strand and which one is the coding strand?

The answer actually depends on which strand the RNA polymerase attaches onto and the direction of the bacterial sigma factor. The sigma factor is a subunit of bacterial polymerase; eukaryotes do not have sigma factors during their transcription process.

Placed in front of the RNA polymerase, the bacterial sigma factor recognizes the promoter region and actually starts transcribing 10 nucleotides. After transcribing 10 nucleotides, the sigma factor latches off and allows the RNA polymerase to move forward to transcribe.

The RNA polymerase continues to transcribe until it reaches a termination signal or sequence, usually the termination sequence is a row of T’s or A’s. To understand why the termination sequence is a row of T’s or A’s, we have to look at their molecular bonds. Nucleotide T and A create the weakest hydrogen bonds, only two hydrogen bonds between each other, compared to C and G, that have three hydrogen bonds. Thus, it is easier to break off from these weaker bonds, and this concept serves ideal for the end of transcription.

As transcription ends, the bacterial sigma factor returns and reattaches to the freed RNA polymerase. Both the RNA polymerase and sigma factor go search for a new promoter to start the transcription process again.

Source: Essential Cell Biology Third Edition