An mRNA 3' processing site targets downstream sequences for rapid degradation in Chlamydomonas chloroplasts

Area of Science:

• Chloroplast gene expression
• RNA processing and maturation
• Molecular biology

Background:

• Chlamydomonas chloroplasts utilize a two-step process for atpB pre-mRNA 3' end maturation.
• This process involves an initial endonuclease cleavage followed by 3' to 5' exonuclease trimming.
• The downstream product of the endonucleolytic cleavage is typically undetectable, suggesting rapid degradation or instability.

Purpose of the Study:

• To investigate the mechanisms underlying the instability of the downstream product of atpB pre-mRNA endonucleolytic processing.
• To determine the roles of the inverted repeat (IR) and endonuclease cleavage site (ECS) in RNA maturation and stability.
• To explore the involvement of exonuclease activities in the degradation pathway.

Main Methods:

• In vivo reporter gene assays using constructs with atpB 3' maturation elements (IR and ECS).
• Systematic deletion and combination of IR and ECS elements to assess their functional synergy.
• Introduction of secondary structures to probe the involvement of 5' to 3' and 3' to 5' exonucleases.

Main Results:

• Constructs containing both IR and ECS prevented reporter gene mRNA accumulation, while IR alone allowed accumulation.
• ECS alone showed an intermediate effect, indicating synergistic action between IR and ECS.
• Degradation was not prevented by structures targeting exonucleases, suggesting rapid endonucleolytic cleavage triggers RNA destruction.
• Fragments of cleaved endogenous atpB mRNA were detected as antisense transcripts.

Conclusions:

• The inverted repeat (IR) and endonuclease cleavage site (ECS) act synergistically to destabilize the downstream product of atpB pre-mRNA cleavage.
• Rapid endonucleolytic cleavages, rather than exonucleases, are primarily responsible for the destruction of the unstable RNA fragment.
• Chloroplast endoribonuclease activities possess evolved mechanisms for selective stabilization of specific cleavage products, similar to 5' mRNA maturation.