Evolution of rubisco complex small subunit transit peptides from algae to plants

Area of Science:

• * Endosymbiotic theory and organelle evolution
• * Molecular biology and protein targeting
• * Evolutionary genomics and bioinformatics

Background:

• * Chloroplasts originated from endosymbiotic cyanobacteria, with most genes transferred to the host nucleus.
• * Nuclear-encoded chloroplast proteins require targeting signals, known as transit peptides (TPs), for import.
• * The evolutionary mechanisms driving TP diversification remain largely uncharacterized.

Purpose of the Study:

• * To investigate the evolutionary trajectory of transit peptides (TPs) in chloroplast-targeted proteins.
• * To compare the functionality of TPs from lower eukaryotes (algae) and higher eukaryotes (plants).
• * To identify key sequence motifs responsible for functional TPs in land plants.

Main Methods:

• * Comparative analysis of transit peptides from Chlamydomonas reinhardtii RbcS (CrRbcS) and Arabidopsis thaliana RbcS (AtRbcS).
• * Functional assays involving the expression of modified CrRbcS TPs in Arabidopsis thaliana.
• * Site-directed mutagenesis to assess the importance of specific sequence motifs (FP-RK, QMMVW) in CrRbcS TP.

Main Results:

• * The Chlamydomonas reinhardtii RbcS (CrRbcS) transit peptide was non-functional in Arabidopsis.
• * Incorporation of the FP-RK motif from Arabidopsis thaliana RbcS (AtRbcS) TP restored chloroplast import function to CrRbcS TP.
• * The QMMVW motif within CrRbcS TP was essential for its transport activity in plants, and its position was critical.
• * Modified CrRbcS TPs with additional plant motifs retained function in Chlamydomonas reinhardtii.

Conclusions:

• * Chloroplast transit peptide evolution involved the acquisition of specific sequence motifs to adapt to host requirements.
• * The FP-RK and QMMVW motifs are crucial for functional chloroplast protein targeting in plants.
• * These findings illuminate the molecular mechanisms underlying the adaptation of algal TPs to plant chloroplasts during land plant evolution.