Page 57 - Cell biology PDC 2024

But how do glucose levels affect the expression of the lac genes?
1- If glucose is present, lactose would not be used.
2- There is a protein called CAP that binds to a site adjacent to the
promoter called CAP site and recruits RNA polymerase to bind the lac
promoter
When glucose is absent, there is an increase in levels of cAMP which
binds to CAP. The CAP- cAMP complex then binds the CAP site.
The CAP binding and the lac repressor dissociating from the operator
when lactose levels are high ensures high levels of transcription of the lac
operon just when it is most needed.

Figure 24: Lac operon activation of expression

2) Regulation of Translation
(1) Micro RNAs (miRNAs)
miRNAs are small non-coding19-25 nucleotides RNAs that are stem-loop
in structure and can regulate translation by binding to 3’-untranslated
sequences of mRNA thus blocking initiation of translation on those
mRNA molecules.The expression rate of the miRNA genes differs in
each tissue and the developmental stage

٥٦