Abstract

BackgroundPyrenoids are protein microcompartments composed mainly of Rubisco that are localized in the chloroplasts of many photosynthetic organisms. Pyrenoids contribute to the CO2-concentrating mechanism. This organelle has been lost many times during algal/plant evolution, including with the origin of land plants. The molecular basis of the evolutionary loss of pyrenoids is a major topic in evolutionary biology. Recently, it was hypothesized that pyrenoid formation is controlled by the hydrophobicity of the two helices on the surface of the Rubisco small subunit (RBCS), but the relationship between hydrophobicity and pyrenoid loss during the evolution of closely related algal/plant lineages has not been examined. Here, we focused on, the Reticulata group of the unicellular green algal genus Chloromonas, within which pyrenoids are present in some species, although they are absent in the closely related species.ResultsBased on de novo transcriptome analysis and Sanger sequencing of cloned reverse transcription-polymerase chain reaction products, rbcS sequences were determined from 11 strains of two pyrenoid-lacking and three pyrenoid-containing species of the Reticulata group. We found that the hydrophobicity of the RBCS helices was roughly correlated with the presence or absence of pyrenoids within the Reticulata group and that a decrease in the hydrophobicity of the RBCS helices may have primarily caused pyrenoid loss during the evolution of this group.ConclusionsAlthough we suggest that the observed correlation may only exist for the Reticulata group, this is still an interesting study that provides novel insight into a potential mechanism determining initial evolutionary steps of gain and loss of the pyrenoid.

Highlights

  • Pyrenoids are protein microcompartments composed mainly of Rubisco that are localized in the chloroplasts of many photosynthetic organisms

  • Based on transformational experiments of the unicellular pyrenoid-containing green alga Chlamydomonas reinhardtii, Meyer et al found that absence of two C. reinhardtii helices of the Rubisco small subunit (RBCS) results in no pyrenoid formation even though the spinach RBCS helices are present in the Rubisco [10]

  • They suggested that hydrophobic interactions between Rubisco molecules within the pyrenoid via two helices on the surface of RBCS are correlated with pyrenoid formation [10]

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Summary

Introduction

Pyrenoids are protein microcompartments composed mainly of Rubisco that are localized in the chloroplasts of many photosynthetic organisms. It was hypothesized that pyrenoid formation is controlled by the hydrophobicity of the two helices on the surface of the Rubisco small subunit (RBCS), but the relationship between hydrophobicity and pyrenoid loss during the evolution of closely related algal/ plant lineages has not been examined. The pyrenoid contributes to the ­CO2-concentrating mechanism [1,2,3,4] This organelle consists mainly of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), a key photosynthetic enzyme composed of eight large and eight small subunits [5]. Based on transformational experiments of the unicellular pyrenoid-containing green alga Chlamydomonas reinhardtii, Meyer et al found that absence of two C. reinhardtii helices of the Rubisco small subunit (RBCS) results in no pyrenoid formation even though the spinach RBCS helices are present in the Rubisco [10]. The relationship between the hydrophobicity of the two RBCS helices and presence or absence of pyrenoids within a closely related lineage has not been studied

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