RESUMO
ADP-glucose pyrophosphorylase (AGPase) and glycogen synthase (GS) catalyze the first two reactions of glycogen synthesis in cyanobacteria. Mutants defective in each of these enzymes in Synechococcus elongatus PCC 7942 were constructed and characterized. Activities of the corresponding enzymes in the selected mutants were virtually undetectable, and their ability to synthesize glycogen was entirely abolished. The maximal activities of photosynthetic O(2) evolution and the rates of respiration in the dark were significantly decreased in the mutants compared to those in wild-type cells. Addition of 0.2 M NaCl or 3 mM H(2)O(2) to liquid cultures markedly inhibited the growth of the AGPase and GS mutants, while the same treatment had only marginal effects on the wild type. These results suggest a significant role for storage polysaccharides in tolerance to salt or oxidative stress.
Assuntos
Metabolismo dos Carboidratos , Glicogênio/biossíntese , Mutação/genética , Synechococcus/enzimologia , Synechococcus/genética , Carboidratos/análise , Clorofila/análise , Clorofila A , Glucose-1-Fosfato Adenililtransferase/genética , Glucose-1-Fosfato Adenililtransferase/metabolismo , Glicogênio Sintase/genética , Glicogênio Sintase/metabolismo , Peróxido de Hidrogênio/farmacologia , Estresse Oxidativo/efeitos dos fármacos , Oxigênio/análise , Fotossíntese/efeitos dos fármacos , Ficocianina/análise , Cloreto de Sódio/farmacologia , Synechococcus/crescimento & desenvolvimentoRESUMO
The putative glgX gene encoding isoamylase-type debranching enzyme was isolated from the cyanobacterium, Synechococcus elongatus PCC 7942. The deduced amino acid sequence indicated that the residues essential to the catalytic activity and substrate binding in bacterial and plant isoamylases and GlgX proteins were all conserved in the GlgX protein of S. elongatus PCC 7942. The role of GlgX in the cyanobacterium was examined by insertional inactivation of the gene. Disruption of the glgX gene resulted in the enhanced fluctuation of glycogen content in the cells during light-dark cycles of the culture, although the effect was marginal. The glycogen of the glgX mutant was enriched with very short chains with degree of polymerization 2 to 4. When the mutant was transformed with putative glgX genes of Synechocystis sp. PCC 6803, the short chains were decreased as compared to the parental mutant strain. The result indicated that GlgX protein contributes to form the branching pattern of polysaccharide in S. elongatus PCC 7942.