Comparing the in Vivo Function of α-Carboxysomes and β-Carboxysomes in Two Model Cyanobacteria

Abstract

The carbon dioxide (CO 2 )-concentrating mechanism of cyanobacteria is characterized by the occurrence of Rubisco-containing microcompartments called carboxysomes within cells. The encapsulation of Rubisco allows for high-CO 2 concentrations at the site of fixation, providing an advantage in low-CO 2 environments. Cyanobacteria with Form-IA Rubisco contain a-carboxysomes, and cyanobacteria with Form-IB Rubisco contain b-carboxysomes. The two carboxysome types have arisen through convergent evolution, and a-cyanobacteria and b-cyanobacteria occupy different ecological niches. Here, we present, to our knowledge, the first direct comparison of the carboxysome function from a-cyanobacteria (Cyanobium spp. PCC7001) and b-cyanobacteria (Synechococcus spp. PCC7942) with similar inorganic carbon (C i ; as CO 2 and HCO 32 ) transporter systems. Despite evolutionary and structural differences between a-carboxysomes and b-carboxysomes, we found that the two strains are remarkably similar in many physiological parameters, particularly the response of photosynthesis to light and external C i and their modulation of internal ribulose-1,5-bisphosphate, phosphoglycerate, and C i pools when grown under comparable conditions. In addition, the different Rubisco forms present in each carboxysome had almost identical kinetic parameters. The conclusions indicate that the possession of different carboxysome types does not significantly influence the physiological function of these species and that similar carboxysome function may be possessed by each carboxysome type. Interestingly, both carboxysome types showed a response to cytosolic C i , which is of higher affinity than predicted by current models, being saturated by 5 to 15 mM C i . This finding has bearing on the viability of transplanting functional carboxysomes into the C 3 chloroplast.

The Minimal CO2-Concentrating Mechanism of Prochlorococcus spp. MED4 Is Effective and Efficient

“…WH5701, also showed no physiological changes when acclimated to different CO 2 concentrations, but some changes in gene expression were observed (Rae et al, 2011). This is not the case for all a-cyanobacteria, as some have strong physiological responses to changes in long-term CO 2 availability Whitehead et al, 2014).…”

“…is in the carboxysome (So et al, 2004) O, as was observed here for Prochlorococcus spp. (Maeda et al, 2002;Whitehead et al, 2014).…”

“…and may explain how the low K P is achieved in the absence of CO 2 recovery mechanisms or high-affinity HCO 3 2 transporters. Other cyanobacteria with a greater repertoire of C i transporters have little excess Rubisco capacity (Whitehead et al, 2014). Once photosynthesis is saturated for CO 2 , further HCO 3 2 import leads to the buildup of the intracellular C i pool and carboxysome CO 2 concentration, such that the rate of CO 2 leakage matches the rate of HCO 3 2 import in excess of photosynthesis.…”

“…Prochlorococcus spp. is a representative of the a-cyanobacteria, a group with distinct CCMs, which have been much less well studied than the CCMs of b-cyanobacteria (Rae et al, 2011(Rae et al, , 2013Whitehead et al, 2014).…”

pH determines the energetic efficiency of the cyanobacterial CO 2 concentrating mechanism

“…It has been suggested that the carboxysome might maintain a pH gradient, with the cytosolic and carboxysomal pHs differing (37,52). Altering the pH in the carboxysome would influence CCM efficiency in two ways.…”