Ribonuclease H

Ribonuclease H structure comparison

RNaseH active site

3puf rnaseH2 AGS mutations

1hys hiv RT highlight rnh

Ribonuclease H (RNase H) is an enzyme that plays a crucial role in the cellular processes of DNA replication, DNA repair, and transcription. It is a type of ribonuclease that specifically degrades the RNA strand of an RNA-DNA hybrid molecule, which is a critical step in various biological mechanisms, including the removal of RNA primers used during DNA replication and the repair of DNA. This enzyme is found in a wide range of organisms, from bacteria to humans, highlighting its essential function in cellular biology.

Function

The primary function of RNase H is to remove RNA segments that are hybridized with DNA. During DNA replication, RNA primers are synthesized by Primase as starting points for DNA polymerase to begin synthesis of the new DNA strand. Once the DNA polymerase has extended the new DNA strand, RNase H removes these RNA primers, allowing DNA polymerase to fill in the gaps with DNA. In the realm of DNA repair, RNase H helps to remove ribonucleotides that are mistakenly incorporated into DNA, preventing potential mutations. Additionally, in the process of transcription, RNase H plays a role in the removal of RNA:DNA hybrids, known as R-loops, which can form and potentially hinder the progression of transcription.

Types

There are two main types of RNase H enzymes, classified based on their structural and functional characteristics: RNase H1 and RNase H2.

• RNase H1 typically recognizes and cleaves the RNA strand of an RNA/DNA hybrid that is at least four ribonucleotides long. It is active as a monomer and does not require any cofactors for its activity.
• RNase H2 is capable of recognizing and cleaving single ribonucleotides embedded in DNA, making it crucial for the repair of misincorporated ribonucleotides. RNase H2 usually functions as part of a complex and requires magnesium ions as cofactors for its activity.

Structure

The structure of RNase H enzymes is characterized by a conserved RNase H domain, which contains the active site responsible for catalysis. This domain is present in a variety of proteins, including those involved in reverse transcription, such as HIV reverse transcriptase. The active site typically contains a series of conserved acidic residues that coordinate the binding of magnesium ions, which are essential for the enzyme's catalytic activity.

Clinical Significance

Mutations in the genes encoding RNase H2 are associated with Aicardi-Goutières syndrome (AGS), a rare genetic disorder that affects the brain, the immune system, and the skin. AGS is characterized by early-onset encephalopathy, which can lead to severe physical and intellectual disability. The involvement of RNase H2 in AGS highlights the enzyme's importance in DNA repair and the maintenance of genomic stability.

Research Applications

RNase H has been exploited in molecular biology research and biotechnology applications. For example, it is used in the removal of RNA templates after cDNA synthesis in the preparation of DNA libraries for next-generation sequencing. Additionally, RNase H's activity is a key component in certain PCR techniques, such as reverse transcription PCR (RT-PCR), where it helps in converting RNA templates into complementary DNA for amplification.