Bacterial Gene Regulation and Transcription Mechanisms

Gene Regulatory Elements

Promoter (P)

Nucleotide sequence where RNA polymerase binds to initiate transcription of a gene or genes.

Structural Genes (GNEs)

Code for protein synthesis involved in the same metabolic process. They are transcribed without interruption, meaning the resultant mRNA carries the information for several proteins (polycistronic mRNA).

Operator (O)

Nucleotide sequence located between the promoter and the structural genes.

Regulatory Gene (R)

Can be part of the bacterial chromosome and encodes the repressor protein.

Transcription Process

Initiation

RNA polymerase recognizes the initiation signal (promoter) and opens the DNA helix, exposing the template strand so it can incorporate ribonucleotides.

Elongation

• RNA polymerase reads the DNA template strand from 3' to 5' and synthesizes RNA from 5' to 3'.
• In eukaryotes, after about 30 nucleotides, a cap (methylguanosine triphosphate) is added to the 5' end; this will be the translation start signal.

Termination

RNA polymerase recognizes a termination sequence in the DNA, signaling the end of transcription.

• Prokaryotes: Often involves a palindromic sequence rich in G-C bases, which causes the formation of a loop structure due to complementarity.
• Eukaryotes: The RNA polymerase transcribes regions longer than the sequence encoding the protein. A specific sequence (e.g., AAUAAA) signals the RNA polymerase to stop. After separation from the DNA, an enzyme adds about 200 adenine nucleotides to the 3' end, forming the poly-A tail (involved in maturation and transport of mRNA outside the nucleus).

Viral Gene Expression Timing

Immediate Early Genes

Transcripts produced before viral DNA synthesis. The encoded proteins block cellular metabolism.

Delayed Early Genes

Encode enzymes like deoxyribonuclease, which hydrolyzes host cell DNA, and provide the nucleotides needed to make viral DNA.

Late Genes

Code for DNA and RNA replication machinery, capsid proteins, and lysozyme, which allows the virus to exit the cell.