cemA homologue essential to CO2 transport in the cyanobacterium Synechocystis PCC6803.

Abstract

We have isolated mutants of Synechocystis PCC6803 that grew very slowly in a low-sodium medium, which is unfavorable for HCO3 transport, and examined two of these mutants (SC1 and SC2) for their ability to take up CO2 and HCO in the light. The COz transport activity of SC1 and SC2 was much lower than that of the wild type (WT), (18) supplemented with 20 mM Hepes-KOH buffer (KOH added to pH 8.0) during aeration with 3% vol/vol CO2 in air. Low-sodium medium was prepared by adding NaCl (final concentration, 100 ,tM) to the BG11 medium in which all of the sodium salts were replaced by potassium salts. Continuous illumination was with tungsten lamps providing 120 tumol of photosynthetically active radiation/m2.s. Cells in a logarithmic phase of growth were mutagenized with N-methyl-N'-nitro-N-nitrosoguanidine (40 ag/ ml) for 20 min in the light, washed twice, and grown with 3% C02/97% air in BG11 medium. Cells were then grown for 2 days with air in the presence of ampicillin (100 ,tg/ml) in the low-Na+ (100 tLM) BG11 medium. The activity of HCO3 uptake in Synechocystis PCC6803 at this Na+ concentration was much lower than at 15 mM Na+ (see curves a and e in Fig. 2C

Absence of light-induced proton extrusion in a cotA-less mutant of Synechocystis sp. strain PCC6803

“…However, no data are available on the function of the cemA gene product (CemA). Both CemA and CotA contain four membrane-spanning domains, and their amino acid sequences are highly conserved, especially in the C-terminal regions (4). These results suggested that these chloroplast and cyanobacterial gene products may have a similar function, and elucidation of the role of CotA is an important step not only in the study of cyanobacterial physiology but also in clarifying the role of CemA in higher plants.…”

Size of cotA and identification of the gene product in Synechocystis sp. strain PCC6803

“…There was no Shine-Dalgarno (26) Comparison of CotA and CemA sequences. As reported in a previous paper (6), the amino acid sequence deduced from the nucleotide sequence of the cotA gene of Synechocystis sp. strain PCC6803 showed significant similarity to the sequences of cemA gene products of various plants (3,15,24,27,30).…”

“…When the first ATG was postulated as the initiation codon, the gene product was a 51-kDa protein of 440 amino acids. We concluded in a previous paper that the translation started from the second ATG codon, giving a gene product of 29 kDa (247 amino acids), close in size to CemA of higher plant chloroplasts (6,24). However, a preliminary experiment to identify the gene product suggested that it may be much larger.…”

“…strain PCC6803 is a homolog of cemA which has been found in chloroplast genomes of various plants (3,6,15,24,29,30). cemA genes of higher plants encode proteins of 229 to 231 amino acids (3,24,29,30), whereas those of liverwort (15) and Chlamydomonas reinhardtii spp.…”

Identification and characterization of a gene encoding a vertebrate-type carbonic anhydrase in cyanobacteria

“…The activity of CCMs results in the energy-dependent transport of both HCO 3 Ϫ and CO 2 across the cytoplasmic membrane and the formation of a large intracellular pool of C i which is used during photosynthesis. Along with as yet unidentified CO 2 /HCO 3 Ϫ transporter proteins and energization components involving NADP(H) dehydrogenasedependent photosystem I electron flow (14,19), a proteinencapsulated aggregate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) called a "carboxysome" is an integral part of a cyanobacterial CCM. It is hypothesized that associated with the cyanobacterial carboxysome is the ␤-type CA, a product of the icfA (also designated ccaA) gene (8,20,27).…”