The Gene ssl3076 Encodes a Protein Mediating the Salt-Induced Expression of ggpS for the Biosynthesis of the Compatible Solute Glucosylglycerol in Synechocystis sp. Strain PCC 6803

Abstract

Acclimation to high salt concentrations involves concerted changes in gene expression. For the majority of salt-regulated genes, the mechanism underlying the induction process is not known. The gene ggpS (sll1566), which encodes the glucosylglycerol-phosphate synthase responsible for the synthesis of the compatible solute glucosylglycerol (GG), is specifically induced by salt in the cyanobacterial model strain Synechocystis sp. strain PCC 6803. To identify mechanisms mediating this salt-specific gene regulation, the ggpS promoter was analyzed in more detail. 5 rapid amplification of cDNA ends (5-RACE) experiments revealed that the adjacent open reading frame (ORF), which is annotated as unknown protein Ssl3076, overlaps with the transcriptional start site of the ggpS gene. Reporter gene expression analyses indicated an essential role for the intact ssl3076 gene in the salt-regulated transcription of a gfp reporter gene. Promoter fragments containing a mutated ssl3076 lost the salt regulation; similarly, a frameshift mutation in ssl3076 resulted in a high level of ggpS expression under low-salt conditions, thereby establishing this small ORF, named ggpR, as a negative regulator of ggpS. Interestingly, small ORFs were also found adjacent to ggpS genes in the genomes of other GG-accumulating cyanobacteria. These results suggest that the GgpR protein represses ggpS expression under low-salt conditions, whereas in salt-shocked and salt-acclimated cells a stress-proportional ggpS expression occurs, leading to GG accumulation.Cyanobacteria represent an evolutionarily old phylum of the domain Bacteria. These organisms are currently attracting much interest due to their global importance as primary producers and as possible sources of renewable bioenergy (1, 6). Cyanobacteria are found in nearly all aquatic systems, including freshwater, marine water, and hypersaline environments. In saline habitats, microorganisms have to cope with two major problems: (i) a permanent ion influx due to a high concentration gradient and (ii) a low external osmotic potential, which causes an efflux of water and loss of turgor pressure (5). Cyanobacteria use the "salt-out" strategy to acclimate to high salt concentrations. Toxic inorganic ions (mainly Na ϩ and Cl Ϫ ) are continuously pumped out from the cytoplasmic space; at the same time, the cells accumulate compatible solutes, which adjust the cellular osmotic potential to levels that allow water uptake (e.g., see references 17 and 40).The freshwater strain Synechocystis sp. strain PCC 6803 (here referred to as PCC 6803) has been established as a model organism for investigating the salt acclimation strategies of cyanobacteria. The main compatible solute accumulated by PCC 6803 is the heteroside glucosylglycerol (GG) (39), which has been found to be characteristic of moderately halotolerant cyanobacteria. GG is obtained either by de novo synthesis or by active uptake from the medium (36). The biosynthesis of GG occurs by a two-step reaction, in which the enzymatic condensation of ADP-glu...

RNA-seq Profiling Reveals Novel Target Genes of LexA in the Cyanobacterium Synechocystis sp. PCC 6803

“…In S.6803, glucosylglycerol (GG) is a major compatible solute to adapt to high-salt or high-osmotic pressure conditions (Klähn and Hagemann, 2011 ). A set of genes related to GG biosynthesis ( ggp, glp ) and uptake ( ggt ) are organized into several gene clusters such as ggtBCD ( slr0529 - 0531 ), ggpS-glpD ( sll1566 - sll1085 ), ggpP-ggtA ( slr0746 - 0747 ), and slr1670 - glpK - spoU - slr1674 - hypA1 ( slr1670 - 1675 ) in S.6803 genome (Mikkat and Hagemann, 2000 ; Klähn et al, 2010 ). RNA-seq analysis revealed that expression levels of these gene clusters were significantly higher in Δ lexA than WT (Table 1 and Table S2 ).…”

“…RNA-seq analysis revealed that expression levels of these gene clusters were significantly higher in Δ lexA than WT (Table 1 and Table S2 ). Klähn et al ( 2010 ) reported that a small ORF, ggpR ( ssl3076 ), exists overlapping with the transcription initiation site of ggpS and its promoter region. Expression of ggpR was also induced by disruption of lexA (Table 1 ).…”

Salt Acclimation of Cyanobacteria and Their Application in Biotechnology

“…During the further salt acclimation, the internal ion content decreases, leading to a stepwise slowdown of GG accumulation. In fully salt-adapted cells, the increased GgpS level [ 103 ] resulting from the ion-mediated release of the small repressor protein GgpR from the ggpS promoter [ 104 ] and the salt-stress-proportional slight increase in Na + and K + guarantee a partially active GgpS pool that is sufficient to produce enough GG in coordination with growth under chronic salt stress [ 102 ]. Currently it is not clear if this elegant salt titration model also applies to other enzymes/proteins involved in salt acclimation.…”

Global transcriptome analysis of salt acclimated Prochlorococcus AS9601

“…In cyanobacteria, salt stress acclimation involves active extrusion of inorganic ions, the accumulation of compatible solutes and the differential expression of many genes (reviewed in Hagemann 2010). The majority of these studies were conducted using marine Synechococcus (Ludwig and Bryant, 2012) and fresh water Synechocystis (Kanesaki et al, 2002;Marin et al, 2003;Paithoonrangsarid et al, 2004;Shoumskaya et al, 2005;Klähn et al, 2010a). Interestingly, studies are not available on the acclimation of Prochlorococcus to salt stress even though it is considered a model organism for cyanobacteria (Partensky et al, 1999).…”