Q.12

Which of the following activities is NOT possessed by the human immunodeficiency

virus-1 reverse transcriptase?

(A) Synthesis of DNA from RNA

(B) Synthesis of DNA from DNA

(C) Degradation of RNA strand of RNA: DNA hybrid

(D) Synthesis of mRNA from DNA

HIV-1 Reverse Transcriptase Functions: Which Activity Does It Lack?

HIV-1 reverse transcriptase (RT) is a key enzyme in the viral life cycle, enabling the virus to convert its RNA genome into DNA for integration into host cells. Understanding its precise activities helps in grasping retroviral replication and antiviral drug design. The correct answer to the query is (D) Synthesis of mRNA from DNA, as this function belongs to host RNA polymerase II, not viral RT.

Core Functions of HIV-1 RT

HIV-1 RT possesses multifunctional enzymatic activities essential for reverse transcription. It synthesizes DNA from an RNA template (RNA-dependent DNA polymerase), marking its hallmark reverse transcriptase activity. RT also performs DNA-dependent DNA polymerase activity, extending DNA primers on DNA templates to complete double-stranded DNA formation.

Additionally, its RNase H domain degrades the RNA strand in RNA:DNA hybrids, clearing the template for second-strand DNA synthesis. These activities ensure efficient conversion of the viral single-stranded RNA genome into proviral DNA.

Option-by-Option Breakdown

• (A) Synthesis of DNA from RNA: This is a primary function of HIV-1 RT, initiating reverse transcription from the viral RNA using a tRNA primer.

• (B) Synthesis of DNA from DNA: RT acts as a DNA polymerase during minus-strand strong-stop DNA extension and plus-strand synthesis on DNA templates.

• (C) Degradation of RNA strand of RNA:DNA hybrid: The integral RNase H activity specifically cleaves RNA in hybrids, critical for processivity and template switching.

• (D) Synthesis of mRNA from DNA: This transcription process is absent in HIV-1 RT; the virus relies on host cellular machinery post-integration for mRNA production.

Implications for HIV Research

Distinguishing RT’s polymerase and RNase H roles from host transcription highlights why nucleoside/nucleotide RT inhibitors target DNA synthesis without affecting mRNA production. This specificity underlies effective antiretroviral therapy.