Problem 5 - Biochemistry

Question

Describe the mechanism by which HIV-1 reverse transcriptase (HIVRT) produces DNA for chromosomal integration.

Step-by-Step Solution

Answer

Answer: HIV-1 reverse transcriptase (HIVRT) plays a crucial role in the replication cycle of HIV-1 by synthesizing a complementary DNA (cDNA) molecule from the viral RNA genome, allowing the viral DNA to integrate into the host's chromosomes and establish a persistent infection. This process involves the initiation of reverse transcription, synthesis of minus-strand and plus-strand DNA, completion of the double-stranded DNA, and ultimately, chromosomal integration.

Step 1: Understanding HIV-1 Reverse Transcriptase

HIV-1 reverse transcriptase (HIVRT) is an essential enzyme for the replication of the human immunodeficiency virus (HIV). It is responsible for synthesizing a complementary DNA (cDNA) molecule from the viral RNA genome, allowing the viral DNA to integrate into the host's chromosomes and establish a persistent infection.

Step 2: Initiation of Reverse Transcription

The process of reverse transcription starts with the binding of HIVRT to the viral RNA genome and a short host-derived RNA primer called tRNA^Lys3. The primer is annealed to an 18-nucleotide sequence known as the primer binding site (PBS) at the 5' end of the viral RNA genome. This binding of the HIVRT, viral RNA, and RNA primer forms the initiation complex for reverse transcription.

Step 3: Synthesis of Minus-Strand DNA

Once the initiation complex is established, HIVRT uses the RNA primer to synthesize the minus-strand DNA, which is complementary to the viral RNA genome. HIVRT moves along the RNA template, adding deoxynucleotide triphosphates (dNTPs) to the 3' end of the primer in a 5' to 3' direction while also continually degrading the RNA template via its RNase H activity. The process continues until the entire viral RNA genome is reverse transcribed into the minus-strand DNA.

Step 4: Plus-Strand DNA Synthesis

After the synthesis of the minus-strand DNA, HIVRT synthesizes plus-strand DNA, which is complementary to the minus-strand DNA. This is initiated by the generation of a second RNA primer known as the polypurine tract (PPT) that is resistant to the RNase H activity of HIVRT. The enzyme then proceeds to synthesize the full-length plus-strand DNA, creating a double-stranded DNA molecule.

Step 5: Completion of the Double-Stranded DNA

To complete the double-stranded DNA molecule, HIVRT fills any gaps in the nucleotide strands, and the remaining RNA primer sequences (tRNA^Lys3 and PPT) are removed. This results in the formation of a linear, double-stranded viral DNA molecule with long terminal repeats (LTRs) at both ends, which is now ready for chromosomal integration.

Step 6: Chromosomal Integration

The final step involves the integration of the viral DNA into the host chromosome, mediated by another viral enzyme called integrase. The integrase cleaves the LTRs at both ends of the viral DNA and facilitates its insertion into the host chromosome. This process ensures the long-term existence of the viral genome within the host cell, allowing for continued viral replication and spread.