Chlamydomonas transcripts encoding three divergent plastid chaperonins are heat-inducible

Area of Science:

• Molecular Biology
• Algal Biology
• Protein Biochemistry

Background:

• Plastid chaperonins, like cpn60, are essential molecular machines involved in protein folding and assembly.
• In higher plants, plastid cpn60 consists of alpha and beta subunits, encoded by distinct genes.
• Understanding chaperonin function in unicellular algae like Chlamydomonas reinhardtii provides insights into chloroplast evolution and stress response.

Purpose of the Study:

• To isolate and characterize cDNAs encoding plastid cpn60 chaperonin subunits in Chlamydomonas reinhardtii.
• To investigate the evolutionary divergence of cpn60 subunits in this green alga.
• To examine the transcriptional regulation of plastid cpn60 genes under heat shock conditions.

Main Methods:

• Isolation and sequencing of three cDNAs encoding plastid cpn60 subunits from Chlamydomonas reinhardtii.
• Comparative analysis of predicted amino acid sequences to determine subunit identity and divergence.
• Quantitative analysis of plastid cpn60 transcript levels under normal and heat shock conditions (42°C).

Main Results:

• Three distinct cDNAs for plastid cpn60 subunits were identified, including alpha and beta types, similar to higher plants.
• Two divergent Chlamydomonas cpn60-beta subunits showed significant sequence differences (24%), suggesting long-term independent evolution.
• Heat shock induced a rapid and substantial increase (10- to 30-fold) in the transcript levels of all three plastid cpn60 genes.

Conclusions:

• Chlamydomonas reinhardtii possesses divergent plastid cpn60-alpha and cpn60-beta subunits, analogous to higher plants.
• The significant divergence in cpn60-beta subunits points to their long-term selective maintenance.
• Unlike higher plants, Chlamydomonas exhibits a strong transcriptional upregulation of plastid cpn60 genes in response to heat stress, indicating a distinct regulatory strategy.