Michal Ronen-Tarazi

Physiological and Molecular Studies on the Response of Cyanobacteria to Changes in the Ambient Inorganic Carbon Concentration

The ability of cyanobacteria to adapt to a wide range of ambient CO2 concentrations involves modulation of the activity of an inorganic carbon-concentrating mechanism (CCM), as well as other changes at various cellular levels including the biosynthetic pathway of purines. Studies of high-CO2-requiring mutants have identified several of the genes involved in the operation of the CCM and in the ability to grow under changing ambient CO2 concentration. In the case of Synechococcus sp. strain PCC 7942 most of these genes have been mapped in the genomic region of the rbcLS operon. Higher levels of detectable transcripts originating from some of these genes have been observed after exposure of the cells to low CO2 concentration. Studies of mutants have confirmed quantitative models postulating crucial roles for carboxysomes and carboxysome-located carbonic anhydrase (CA) in cyanobacterial photosynthesis. A central role is also proposed for cytoplasmic-membrane-associated CA activity: CA may function to scavenge escaping CO2 by intracellular conversion to bicarbonate against the chemical potential.

A putative HCO−3 transporter in the cyanobacterium Synechococcus sp. strain PCC 79421

Cyanobacteria possess an inducible mechanism which enables them to concentrate inorganic carbon (Ci) within the cells. An inactivation library was used to raise the high‐CO2‐requiring mutant of Synechococcus PCC 7942, IL‐2, impaired in HCO− 3 transport. Analysis of the relevant genomic DNA detected several modifications, probably due to the single crossover recombination, leading to inactivation of ORF467 (designated ictB) in IL‐2. IctB contains 10 trans‐membrane regions and is homologous to several transport‐related proteins from various organisms. Kinetic analyses of HCO− 3 uptake in the wild type and IL‐2 suggested the presence of two or three HCO− 3 carriers exhibiting different affinities to HCO− 3.

The genomic region of rbcLS in Synechococcus sp. PCC 7942 contains genes involved in the ability to grow under low CO2 concentration and in chlorophyll biosynthesis.

Several genes involved in the ability of Synechococcus sp. PCC 7942 to grow under different CO2 concentrations were mapped in the genomic region of rbcLS (the operon encoding the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase). Insertion of a cartridge encoding kanamycin resistance within open reading frame (ORF) 78, designated ccmJ, located 7 kb upstream of rbcLS, resulted in a kanamycin-resistant, high-CO2-requiring mutant, M3, which does not contain normal carboxysomes. ccmJ shows significant homology to csoS1 encoding a carboxysomal shell polypeptide in Thiobacillus neopolitanus. Analysis of the polypeptide pattern of a carboxysome-enriched fraction indicated several differences between the wild type and the mutant. The amount of the ribulose-1,5-bisphosphate carboxylase/oxygenase subunits was considerably smaller in the carboxysomal fraction of the mutant when compared to the wild type. On the basis of the sequence analyses, ORF286 and ORF466, located downstream of ccmJ, were identified as chlL and chlN, respectively, which are involved in chlorophyll biosynthesis in the dark.

Response of Photosynthetic Microorganisms to Changing Ambient Concentration of CO2

In this manuscript we briefly describe three aspects of our recent studies on the response of cyanobacteria to changes in ambient level of carbon dioxide. We identified several genes responding and used one of them to initiate study on the regulation of gene expression by CO2. We also developed a method for the isolation of new mutants which demand high-CO2 for growth and for the identification of the genes involved. These cells are being used for the elucidation of the biotic and environmental factors affecting the fractionation of stable carbon isotopes, a parameter widely used to assess the level of CO2 during geological periods.

Cyanobacterial Mutants Defective in HCO 3 - Uptake

Photosynthetic microorganisms are capable of adapting to a wide range of CO2 concentrations. When cyanobacterial cells are transferred from high to low concentrations of CO2, they undergo an adaptation process. This includes modulation of the expression of certain genes, some of which are involved in the operation of the inorganic carbon (Ci)-concentrating mechanism (CCM, see Miller et al. 1990; Kaplan et al. 1991; Raven 1991; Badger, Price 1992; Ogawa 1993 and Kaplan et al. 1994 for recent reviews and literature citations). The active accumulation of Ci to levels as high as 50-100 mM Ci, consequent on the activity of the CCM, enables the cells to perform efficient photosynthesis in spite of the relatively low affinity of their ribulose 1,5- bisphosphate carboxylase (rubisco) for CO2. Moreover, the elevated concentration of CO2 in close proximity to rubisco, within the carboxysomes, activates the enzyme, reduces competition by O2 and inhibits photorespiration (Schwarz et al. 1995)

The fluxes of inorganic carbon and CO2-dependent genes involved in the cyanobacterial inorganic carbon-concentrating mechanism: A view on some of the open questions

In this manuscript we briefly discuss some of the open questions in relation to the response of photosynthetic microorganisms to changes in the ambient concentration of CO2. We focus on the regulation and activity of the inorganic carbon-concentrating mechanism (CCM) in cyanobacteria and its contribution to the fractionation of stable carbon isotopes (δ13C).

The Inorganic Carbon-Concentrating Mechanism of Cyanobacteria

In this chapter we briefly present and discuss recent progress in the elucidation of certain physiological and molecular aspects of the cyanobacterial inorganic carbon (Ci)-concentrating mechanism (CCM). The reader is referred to earlier chapters and reviews [1—14] for a comprehensive account of other important aspects, including the acclimation of cyanobacteria to changing CO2 concentration.