The ggtA gene encodes a subunit of the transport system for the osmoprotective compound glucosylglycerol in Synechocystis sp. strain PCC 6803

The ggtA gene was sequenced during the analysis of a mutant of Synechocystis sp. strain PCC 6803 with impaired salt tolerance. It showed striking sequence similarities to ATP-binding proteins of binding-protein-dependent transport systems (ABC transporters). Mutants of ggtA and three neighboring reading frames were constructed by inserting an aphII gene cassette and were physiologically and genetically characterized. The ggtA insertion mutant lost its glucosylglycerol (GG) uptake ability, but its salt tolerance did not change. Therefore, it was concluded that active transport of the osmoprotective compound GG in Synechocystis is mediated by an ABC transporter. The genes for the GG-specific ABC transporter are not organized in an operon as usually found for comparable transporters, since the other insertion mutants showed normal GG transport activity. After cultivation of the ggtA mutant at high salt concentrations, significant amounts of GG were found in the cultivation medium, indicating that GG transport is mainly necessary for recovery of GG leaked through the cytoplasmic membrane. The Northern blot technique revealed increased transcription of the ggtA gene in cells adapted to higher salt concentrations, whereas in cells from basal medium, its transcription was weak.

Uptake and use of the osmoprotective compounds trehalose, glucosylglycerol, and sucrose by the cyanobacterium Synechocystis sp. PCC6803

Accumulation of exogenously supplied osmoprotective compounds was analyzed in the cyanobacterium Synechocystis sp. PCC6803, which synthesizes glucosylglycerol as the principal osmoprotective compound. Glucosylglycerol and trehalose were accumulated to high levels and protected cells of a mutant unable to synthesize glucosylglycerol against the deleterious effects of salt stress. In the wild-type, uptake of trehalose repressed the synthesis of glucosylglycerol and caused metabolic conversion of originally accumulated glucosylglycerol. Trehalose cannot be synthesized by Synechocystis and was not or only insignificantly metabolized. Sucrose, which can be synthesized in low quantities by Synechocystis, was also taken up, as indicated by its disappearance from the medium. Sucrose was not accumulated to high levels, probably due to a sucrose-degrading activity found in cells adapted to both low- and high-salt conditions. Despite its low intracellular concentration, sucrose showed a weak osmoprotective effect in salt-shocked cells of a mutant unable to synthesize glucosylglycerol.

Characterization of a glucosylglycerol-phosphate-accumulating, salt-sensitive mutant of the cyanobacterium Synechocystis sp. strain PCC 6803

Salt-sensitive mutants of Synechocystis were obtained by random cartridge mutagenesis, and one mutant (mutant 4) was characterized in detail. The salt tolerance of mutant 4 was reduced to about 20% of that of the wild-type. This was caused by a defect in the biosynthetic pathway of the osmoprotective compound glucosylglycerol (GG). Salt-treated cells of mutant 4 accumulated the intermediate glucosylglycerol-phosphate (GG-P). Only low levels of phosphate-free GG were detected. The phosphorylated form of GG was not osmoprotective and seemed to be toxic. In vitro enzyme assays revealed that GG-P-phosphatase activity was completely absent in mutant 4, while GG-P-synthase remained unchanged. The integration site of the aphII cartridge in mutant 4 and the corresponding wild-type region was cloned and sequenced. Mutant 4 was complemented to salt resistance after transformation by the cloned wild-type region. The integration of the cartridge led to a deletion of about 1.1 kb of the chromosomal DNA. This affected two of the identified putative protein coding regions, orfII and stpA. The ORFII protein shows a high degree of similarity to the receiver domain of response regulator proteins. Related sequences were not found for StpA. We assume that in mutant 4, regulatory genes necessary for the process of salt adaptation in Synechocystis are impaired.

Active transport of glucosylglycerol is involved in salt adaptation of the cyanobacterium Synechocystis sp. strain PCC 6803

An active-transport system for the osmoprotective compound glucosylglycerol (GG) was found in the cyanobacterium Synechocystis sp. strain PCC 6803. Uptake assays with 14C-labelled GG showed that the GG transport was enhanced in cells adapted to increasing concentrations of NaCl. Kinetic studies indicated a Michaelis-Menten relationship. The uptake of GG was energy dependent and occurred against a steep concentration gradient. It was inhibited by uncouplers as well as by a combination of darkness and KCN. The affinity of the transporter seems to be restricted to osmoprotective compounds of cyanobacteria; from a variety of compounds tested only sucrose and trehalose competed with GG for uptake. A salt-sensitive mutant of Synechocystis 6803 unable to synthesize GG could be complemented to salt resistance by exogenous GG. Accumulation of GG from the medium was essential for the restoration of photosynthesis and growth in mutant cells under high-salt conditions. In wild-type cells, the GG transporter probably serves to prevent GG leaking out of salt-stressed cells.

Cognitive and family-support mediators of preschool effectiveness: a confirmatory analysis

Investigated in this study were the mediators of the effects of preschool intervention on children's school achievement in sixth grade. A confirmatory structural model developed in a previous study of third graders was tested with 360 low-income, mostly black children who were available at the 3-year follow-up. The model incorporated cognitive readiness at kindergarten entry and parent involvement in school (rated by teachers and parents) as primary mediators of preschool effectiveness. In sixth grade (age 12), preschool participation at ages 3 or 4 was significantly associated with higher reading achievement, higher math achievement, and with lower incidence of grade retention. Cognitive readiness and parent involvement in school significantly mediated the estimated effects of preschool participation on school achievement and grade retention 7 years postprogram. Teacher ratings of school adjustment, school mobility, and grade retention also contributed to the transmission of effects. This integrated model fit the data better than several alternative models, including those based on the cognitive-advantage and family-support hypotheses.