Dynamics of carbon-concentrating mechanism induction and protein relocalization during the dark-to-light transition in synchronized Chlamydomonas reinhardtii.

Madeline C Mitchell,Howard Griffiths,Moritz T Meyer

doi:10.1104/pp.114.246918

Abstract

In the model green alga Chlamydomonas reinhardtii, a carbon-concentrating mechanism (CCM) is induced under low CO2 in the light and comprises active inorganic carbon transport components, carbonic anhydrases, and aggregation of Rubisco in the chloroplast pyrenoid. Previous studies have focused predominantly on asynchronous cultures of cells grown under low versus high CO2. Here, we have investigated the dynamics of CCM activation in synchronized cells grown in dark/light cycles compared with induction under low CO2. The specific focus was to undertake detailed time course experiments comparing physiology and gene expression during the dark-to-light transition. First, the CCM could be fully induced 1 h before dawn, as measured by the photosynthetic affinity for inorganic carbon. This occurred in advance of maximum gene transcription and protein accumulation and contrasted with the coordinated induction observed under low CO2. Between 2 and 1 h before dawn, the proportion of Rubisco and the thylakoid lumen carbonic anhydrase in the pyrenoid rose substantially, coincident with increased CCM activity. Thus, other mechanisms are likely to activate the CCM before dawn, independent of gene transcription of known CCM components. Furthermore, this study highlights the value of using synchronized cells during the dark-to-light transition as an alternative means of investigating CCM induction.

Highlights

In the model green alga Chlamydomonas reinhardtii, a carbon-concentrating mechanism (CCM) is induced under low CO2 in the light and comprises active inorganic carbon transport components, carbonic anhydrases, and aggregation of Rubisco in the chloroplast pyrenoid
In the model green alga Chlamydomonas reinhardtii, three elements are important for CCM activity: first, inorganic carbon (Ci) transporters at the plasma membrane and chloroplast envelope (Spalding, 2008); second, carbonic anhydrases, which facilitate the interconversion of CO2 and HCO3– and operate in parallel within each cellular compartment (Moroney et al, 2011); and third, the localization of Rubisco to a chloroplast microcompartment called the pyrenoid to minimize CO2
There are similarities in the transcriptional response of genes during CCM induction in response to low CO2 and during the dark-to-light transition, this study has identified several key differences that distinguish the response of synchronized cells from that of asynchronous cultures

Summary

Introduction

In the model green alga Chlamydomonas reinhardtii, a carbon-concentrating mechanism (CCM) is induced under low CO2 in the light and comprises active inorganic carbon transport components, carbonic anhydrases, and aggregation of Rubisco in the chloroplast pyrenoid. The CCM could be fully induced 1 h before dawn, as measured by the photosynthetic affinity for inorganic carbon This occurred in advance of maximum gene transcription and protein accumulation and contrasted with the coordinated induction observed under low CO2. In the model green alga Chlamydomonas reinhardtii, three elements are important for CCM activity: first, inorganic carbon (Ci) transporters at the plasma membrane and chloroplast envelope (Spalding, 2008); second, carbonic anhydrases, which facilitate the interconversion of CO2 and HCO3– and operate in parallel within each cellular compartment (Moroney et al, 2011); and third, the localization of Rubisco to a chloroplast microcompartment called the pyrenoid to minimize CO2. CAH3 encodes a thylakoidlumenal carbonic anhydrase essential for growth at ambient CO2, the gene is minimally CO2

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