recap

16.3 recap

A virus consists of nucleic acids, a few proteins, and in some cases, a lipid envelope. Viruses require host cells to reproduce. Viral life cycles can include lytic and lysogenic stages. Bacteriophages λ use both positive and negative regulators of transcription initiation. Studies of HIV revealed a new mechanism for gene regulation: the regulation of transcription elongation.

learning outcomes

You should be able to:

  • Identify instances of positive and negative regulation of viral gene expression in prokaryotes.

  • Predict how an anti-HIV drug would work at the molecular level.

Question 1

Describe positive and negative regulation of gene expression in the bacteriophage λ and HIV life cycles.

In the bacteriophage: Phage DNA is injected into the host cell. Early phase genes are transcribed at phage promoters by using DNA sequences similar to those of the host cell (positive regulation). This leads to early protein that binds to host promoters to shut them down (negative regulation). Other early proteins lead host RNA polymerase to transcribe middle and late phage genes (positive regulation).

In HIV: HIV RNA is injected into the host cell. HIV reverse transcriptase is activated to make cDNA and integrase to splice cDNA into the host chromosome (positive regulation). Later, host RNA polymerase binds to HIV promoters to make HIV mRNAs (positive regulation). HIV tat protein acts as an anti-terminator for the transcription of HIV genes integrated into the host genome.

Question 2

If the function of the protease that cuts HIV protein was specifically blocked by a drug, what would be the effect on the HIV reproductive cycle?

HIV infection, cDNA formation and integration would occur as normal. Viral genes would be expressed and a large precursor protein would be made. But it would not be cut into separate viral proteins. HIVE particles would not be packaged or released from the cell.

349

So far we have discussed mechanisms that cells and viruses use to control gene transcription. These mechanisms usually involve the interaction of regulatory proteins with specific DNA sequences. However, there are other mechanisms for controlling gene expression that do not depend on specific DNA sequences. We will discuss these mechanisms in the next section.