Transfer of chloroplast <i>rbcL</i> and <i>atpB</i> genes to nucleus enables partial rescue of photoautotrophic growth of Chlamydomonas.
Transferring chloroplast genes atpB and rbcL to the nucleus can support some photoautotrophic growth in Chlamydomonas, but the low protein levels limit full functionality.
Where it sits
this study against the rest of the ghrp-6 corpusSummary and findings
This study examined the transfer of chloroplast genes atpB and rbcL to the nucleus of Chlamydomonas reinhardtii to assess their impact on photoautotrophic growth. The atpB-TN strains maintained substantial photoautotrophic growth, while the rbcL-TN strains only grew photoautotrophically under 5% CO2. Protein levels for ATPB and RBCL were reported at 7-10% and less than 6% of wild-type levels, respectively.
Abstract
An ancient cyanobacterium has evolved through endosymbiosis to form extant chloroplasts of eukaryotic algae and higher plants. During this process, most genes have been transferred to the nuclear genomes. The chloroplast gene sets of different photosynthetic species are relatively conserved yet still exhibiting differences among different species, suggesting a dynamic process and divergence of chloroplast-to-nucleus gene transfer events. Here, based on a comparison of 15 representative green lineage species with sequenced chloroplast genomes, we selected two genes, <i>atpB</i> and <i>rbcL</i>, from a set of 41 conserved genes for chloroplast-to-nucleus gene transfer test. Using the green alga <i>Chlamydomonas reinhardtii</i> as a model organism, we expressed these two genes in the nuclear genome in the corresponding chloroplast mutant background. We demonstrate that transferring <i>atpB</i> and <i>rbcL</i> to the nucleus sustains photoautotrophic growth at different levels. The <i>atpB</i>-TN (transfer-to-nucleus) strains retained photoautotrophic growth to a substantial extent, whereas the <i>rbcL</i>-TN strains were able to grow photoautotrophically only under 5% CO<sub>2</sub> and not under ambient air (∼0.04% CO<sub>2</sub>). The nucleus-encoded proteins accumulated to levels substantially lower than those of the chloroplast-expressed proteins, reaching only 7∼10% (for ATPB) and less than 6% (for RBCL) of the wild-type level. Although both two proteins were relocalized to the chloroplast and assembled into respective ATP synthase and Rubisco complexes, their low abundance appears to account for the incomplete restoration of photosynthetic capability. This study provides a framework for progressively transferring more chloroplast protein-coding genes to the nucleus towards achieving a minimal chloroplast genome in a green algal chassis.
Background
This paper addresses the evolutionary process of gene transfer from chloroplasts to the nucleus in eukaryotic algae and higher plants. Prior research has established that most chloroplast genes have been relocated to nuclear genomes, yet the dynamics of this transfer remain unclear. Understanding this process is essential for manipulating algal genomes to enhance photosynthetic efficiency.
Methods
The study utilized Chlamydomonas reinhardtii as a model organism, focusing on the transfer of two chloroplast genes, atpB and rbcL, to the nuclear genome. The specific experimental design and population details are not reported in the abstract. The primary outcome measures were the growth capabilities of the modified strains under varying CO2 conditions.
Results
The primary observation was that atpB-TN strains maintained photoautotrophic growth, while rbcL-TN strains could only grow photoautotrophically under 5% CO2. The expression levels of the transferred proteins were significantly lower than those expressed in chloroplasts, with ATPB at 7∼10% and RBCL at less than 6% of wild-type levels.
Interpretation
The results suggest that while the transfer of chloroplast genes can sustain some level of photosynthetic growth, the low expression levels of the proteins limit the restoration of full photosynthetic capability. This aligns with previous literature indicating challenges in gene expression following such transfers. The small effect sizes may not translate to clinically meaningful improvements in photosynthetic efficiency, and the study's limitations, such as lack of detailed methodology and population data, hinder broader conclusions.
Key findings
- atpB-TN strains retained photoautotrophic growth to a substantial extent.
- rbcL-TN strains were able to grow photoautotrophically only under 5% CO2 and not under ambient air (∼0.04% CO2).
- nucleus-encoded ATPB proteins reached only 7∼10% of the wild-type level.
- nucleus-encoded RBCL proteins reached less than 6% of the wild-type level.
Limitations
- Not reported in abstract.
- Lack of detailed methodology and population data.