Practice question

Gene expression in prokaryotes initiates when RNA polymerase holoenzyme containing sigma factor recognizes and binds to promoter elements -35 and -10 upstream of transcription start site, melting DNA and initiating RNA synthesis. Cloning vectors must supply a compatible promoter such as lac, trc, T7, or tac oriented to drive transcription of inserted coding sequence. Origin of replication ensures plasmid maintenance, antibiotic gene provides selection, and lacZ offers screening; none direct transcription of cloned gene. Hence promoter strength and regulation determine mRNA abundance and subsequent recombinant protein yield.