Retron reverse transcriptase termination and phage defense are dependent on host RNase H1

Area of Science:

• Molecular Biology
• Bacteriology
• Genetics

Background:

• Retrons are bacterial retroelements producing reverse-transcribed DNA (RT-DNA) essential for phage defense.
• Retron RT-DNA synthesis involves a unique 2'-5' initiation and a poorly understood termination mechanism.
• Retron reverse transcriptases (RTs) lack RNase H domains, necessitating host RNase H1 for RNA template removal.

Purpose of the Study:

• To elucidate the role of RNase H1 in retron RT-DNA termination.
• To investigate how RNase H1 influences the precise termination of retron-derived DNA.
• To understand the impact of RNase H1 on retron-mediated bacterial defense against phages.

Main Methods:

• In vitro biochemical assays using a reduced system to study retron RT-DNA termination.
• Comparative analysis of RT-DNA termination across different retron subtypes.
• Genetic analysis of retron-based phage defense in the presence and absence of RNase H1.

Main Results:

• RNase H1 is crucial for determining the termination point of retron RT-DNA, with varying effects across retron subtypes.
• The termination mechanism relies on the tertiary structure of RT-DNA and RNA, not steric hindrance or RNA secondary structure.
• Retron-based phage defense is significantly weakened in cells lacking functional RNase H1.

Conclusions:

• RNase H1 has an expanded role in retron biology, actively controlling RT-DNA termination beyond simple template removal.
• A model involving the tertiary structure of nucleic acid intermediates explains retron RT-DNA termination.
• RNase H1 is a critical host factor for effective retron-mediated bacterial immunity against phages.