Latent nitrate transport activity of a novel sulfate permease-like protein of the cyanobacterium Synechococcus elongatus

Availability of vitamin B12 and its lower ligand intermediate α-ribazole impact prokaryotic and protist communities in oceanic systems

Genome analyses predict that the cofactor cobalamin (vitamin B₁₂, called B₁₂ herein) is produced by only one-third of all prokaryotes but almost all encode at least one B₁₂-dependent enzyme, in most cases methionine synthase. This implies that the majority of prokaryotes relies on exogenous B₁₂ supply and interacts with producers. B₁₂ consists of a corrin ring centred around a cobalt ion and the lower ligand 5’6-dimethylbenzimidazole (DMB). It has never been tested whether availability of this pivotal cofactor, DMB or its intermediate α-ribazole affect growth and composition of prokaryotic microbial communities. Here we show that in the subtropical, equatorial and polar frontal Pacific Ocean supply of B₁₂ and α-ribazole enhances heterotrophic prokaryotic production and alters the composition of prokaryotic and heterotrophic protist communities. In the polar frontal Pacific, the SAR11 clade and Oceanospirillales increased their relative abundances upon B₁₂ supply. In the subtropical Pacific, Oceanospirillales increased their relative abundance upon B₁₂ supply as well but also downregulated the transcription of the btuB gene, encoding the outer membrane permease for B₁₂. Surprisingly, Prochlorococcus, known to produce pseudo-B₁₂ and not B₁₂, exhibited significant upregulation of genes encoding key proteins of photosystem I + II, carbon fixation and nitrate reduction upon B₁₂ supply in the subtropical Pacific. These findings show that availability of B₁₂ and α-ribazole affect growth and composition of prokaryotic and protist communities in oceanic systems thus revealing far-reaching consequences of methionine biosynthesis and other B₁₂-dependent enzymatic reactions on a community level.

A Novel Bacterial Nitrate Transporter Composed of Small Transmembrane Proteins

A putative silent gene of the freshwater cyanobacterium Synechococcus elongatus strain PCC 7942, encoding a small protein with two transmembrane helices, was named nrtS, since its overexpression from an inducible promoter conferred nitrate uptake activity on the nitrate transport-less NA4 mutant of S. elongatus. Homologs of nrtS, encoding proteins of 67-118 amino acid residues, are present in a limited number of eubacteria including mostly cyanobacteria and proteobacteria, but some others, e.g. the actinobacteria of the Mycobacterium tuberculosis complex, also have the gene. When expressed in NA4, the nrtS homolog of the γ-proteobacterium Marinomonas mediterranea took up nitrate with higher affinity for the substrate as compared with the S. elongatus NrtS (Km of 0.49 mM vs. 2.5 mM). Among the 61 bacterial species carrying the nrtS homolog, the marine cyanobacterium Synechococcus sp. strain PCC 7002 is unique in having two nrtS genes (nrtS1 and nrtS2) located in tandem on the chromosome. Coexpression of the two genes in NA4 resulted in nitrate uptake with a Km (NO3-) of 0.15 mM, while expression of either of the two resulted in low-affinity nitrate uptake activity with Km values of >3 mM, indicating that NrtS1 and NrtS2 form a heteromeric transporter complex. The heteromeric transporter was shown to transport nitrite as well. A Synechococcus sp. strain PCC 7002 mutant defective in the nitrate transporter (NrtP) showed a residual activity of nitrate uptake, which was ascribed to the NrtS proteins. Blue-native PAGE and immunoblotting analysis suggested a hexameric structure for the NrtS proteins.

The SLC26 gene family of anion transporters and channels

The phylogenetically ancient SLC26 gene family encodes multifunctional anion exchangers and anion channels transporting a broad range of substrates, including Cl(-), HCO3(-), sulfate, oxalate, I(-), and formate. SLC26 polypeptides are characterized by N-terminal cytoplasmic domains, 10-14 hydrophobic transmembrane spans, and C-terminal cytoplasmic STAS domains, and appear to be homo-oligomeric. SLC26-related SulP proteins of marine bacteria likely transport HCO3(-) as part of oceanic carbon fixation. SulP genes present in antibiotic operons may provide sulfate for antibiotic biosynthetic pathways. SLC26-related Sultr proteins transport sulfate in unicellular eukaryotes and in plants. Mutations in three human SLC26 genes are associated with congenital or early onset Mendelian diseases: chondrodysplasias for SLC26A2, chloride diarrhea for SLC26A3, and deafness with enlargement of the vestibular aqueduct for SLC26A4. Additional disease phenotypes evident only in mouse knockout models include oxalate urolithiasis for Slc26a6 and Slc26a1, non-syndromic deafness for Slc26a5, gastric hypochlorhydria for Slc26a7 and Slc26a9, distal renal tubular acidosis for Slc26a7, and male infertility for Slc26a8. STAS domains are required for cell surface expression of SLC26 proteins, and contribute to regulation of the cystic fibrosis transmembrane regulator in complex, cell- and tissue-specific ways. The protein interactomes of SLC26 polypeptides are under active investigation.

Localisation of a candidate anion transporter to the surface of the malaria parasite

PfSulP, encoded by the human malaria parasite Plasmodium falciparum, is a member of the sulphate permease family of anion transporter proteins. By transfecting the parasite with an epitope-tagged version of PfSulP, and detecting via western blot and indirect immunofluorescent assay microscopy, we show that PfSulP is localised to the surface of the intraerythrocytic parasite, where it is postulated to play a role in the flux of anions across the parasite plasma membrane.

Complete nucleotide sequence of the freshwater unicellular cyanobacterium Synechococcus elongatus PCC 6301 chromosome: gene content and organization

The entire genome of the unicellular cyanobacterium Synechococcus elongatus PCC 6301 (formerly Anacystis nidulans Berkeley strain 6301) was sequenced. The genome consisted of a circular chromosome 2,696,255 bp long. A total of 2,525 potential protein-coding genes, two sets of rRNA genes, 45 tRNA genes representing 42 tRNA species, and several genes for small stable RNAs were assigned to the chromosome by similarity searches and computer predictions. The translated products of 56% of the potential protein-coding genes showed sequence similarities to experimentally identified and predicted proteins of known function, and the products of 35% of the genes showed sequence similarities to the translated products of hypothetical genes. The remaining 9% of genes lacked significant similarities to genes for predicted proteins in the public DNA databases. Some 139 genes coding for photosynthesis-related components were identified. Thirty-seven genes for two-component signal transduction systems were also identified. This is the smallest number of such genes identified in cyanobacteria, except for marine cyanobacteria, suggesting that only simple signal transduction systems are found in this strain. The gene arrangement and nucleotide sequence of Synechococcus elongatus PCC 6301 were nearly identical to those of a closely related strain Synechococcus elongatus PCC 7942, except for the presence of a 188.6 kb inversion. The sequences as well as the gene information shown in this paper are available in the Web database, CYORF (http://www.cyano.genome.jp/).

A New Class of Signal Transducer in His-Asp Phosphorelay Systems

Nitrate transport activity of the LtnT permease of the cyanobacterium Synechococcus elongatus is activated when LtnA, a response regulator without an effector domain, is phosphorylated by LtnB, a hybrid histidine kinase. We identified a protein (LtnC) that is required for activation of LtnT. LtnC consists of an N-terminal histidine-containing phosphoacceptor (HisKA) domain, a receiver domain, and a unique C-terminal domain found in some cyanobacterial proteins. Because LtnC lacks an ATP-binding kinase domain of a histidine kinase, it is incapable of autophosphorylation, but LtnC is phosphorylated by LtnA. The histidine residue in the HisKA domain but not the aspartate residue in the receiver domain is essential for phosphorylation of LtnC and activation of LtnT. LtnC phosphorylation leads to oligomerization of the protein. Fusion of the C-terminal domain of LtnC to glutathione S-transferase, which forms oligomers, also activates LtnT, suggesting that oligomerization of the LtnC C-terminal domain causes LtnT activation. These results indicate that the C-terminal domain of LtnC acts as an effector domain that directs the output of the signal from the phosphorelay system. The two-step (His-Asp-His) phosphorelay system, composed of the LtnB, LtnA, and LtnC proteins, is distinct from the known phosphorelay systems, namely, the typical two-component system (His-Asp) and the multistep phosphorelay system (His-Asp-His-Asp), because the HisKA domain of LtnC is the terminal phosphoacceptor that determines the signal output. LtnC is a new class of signal transducer in His-Asp phosphorelay systems that contains a HisKA domain and an effector domain.