Search

Chlorophyllase catalyzes the first step in the catabolic pathway of chlorophyll. It is a constitutive enzyme located in chloroplast membranes. In isolated plastids the hydrolysis of the endogenous chlorophyll does not take place unless the membranes are solubilized in the presence of detergent. The structural latency of chlorophyllase activity appears to be due to the differential locations of substrate and enzyme within the plastids. Envelope membranes prepared from both chloroplasts and gerontoplasts contain chlorophyllase activity. The isolation of envelopes is associated with a marked increase in chlorophyllase activity per unit of protein. Yields of chlorophyllase and of specific envelope markers in the final preparations are similar, suggesting that the enzyme may be located in the envelope. It is hypothesized that the breakdown of chlorophyll during leaf senescence requires a mechanism that mediates the transfer of chlorophyll from the thylakoidal pigment-protein complexes to the sites of catabolic reactions in the envelope

The analysis of gene expression in Arabidopsis thaliana using cDNA-microarrays and RT-PCR showed that AtOSA1 (Arabidopsis thaliana Oxidative Stress related Abc1 like protein) transcript levels are influenced by Cd²⁺ treatment. The comparison of protein sequences revealed that AtOSA1 belongs to the family of Abc1 proteins. Up to now, Abc1-like proteins have been identified in prokaryotes and in the mitochondria of eukaryotes. AtOSA1 is the first member of this family to be localized in the chloroplasts. However, despite sharing homology to the mitochondrial ABC1 of Saccharomyces cerevisiae, AtOSA1 was not able to complement yeast strains deleted in endogenous ABC1 gene, thereby suggesting different function between AtOSA1 and the yeast ABC1.The atosa1-1 and atosa1-2 T-DNA insertion mutants were more affected than wild type plants by Cd²⁺ and revealed an increased sensitivity towards oxidative stress (H₂O₂) and high light. The mutants exhibited higher superoxide dismutase activities and differences in the expression of genes involved in the antioxidant pathway.In addition to the conserved Abc1 region in the AtOSA1 protein sequence, putative kinase domains were found. Protein kinase assays in gelo using myelin basic protein as a kinase substrate revealed that chloroplast envelope membrane fractions from the AtOSA1 mutant lacked a 70 kD phosphorylated protein compared to the wild type. Our data suggest that the chloroplast AtOSA1 protein is a new factor playing a role in a balance of oxidative stress.

The analysis of gene expression in Arabidopsis thaliana using cDNA-microarrays and RT-PCR showed that AtOSA1 (Arabidopsis thaliana Oxidative Stress related Abc1 like protein) transcript levels are influenced by Cd²⁺ treatment. The comparison of protein sequences revealed that AtOSA1 belongs to the family of Abc1 proteins. Up to now, Abc1-like proteins have been identified in prokaryotes and in the mitochondria of eukaryotes. AtOSA1 is the first member of this family to be localized in the chloroplasts. However, despite sharing homology to the mitochondrial ABC1 of Saccharomyces cerevisiae, AtOSA1 was not able to complement yeast strains deleted in endogenous ABC1 gene, thereby suggesting different function between AtOSA1 and the yeast ABC1.The atosa1-1 and atosa1-2 T-DNA insertion mutants were more affected than wild type plants by Cd²⁺ and revealed an increased sensitivity towards oxidative stress (H₂O₂) and high light. The mutants exhibited higher superoxide dismutase activities and differences in the expression of genes involved in the antioxidant pathway.In addition to the conserved Abc1 region in the AtOSA1 protein sequence, putative kinase domains were found. Protein kinase assays in gelo using myelin basic protein as a kinase substrate revealed that chloroplast envelope membrane fractions from the AtOSA1 mutant lacked a 70 kD phosphorylated protein compared to the wild type. Our data suggest that the chloroplast AtOSA1 protein is a new factor playing a role in a balance of oxidative stress.

Chlorophyllase catalyzes the first step in the catabolic pathway of chlorophyll. It is a constitutive enzyme located in chloroplast membranes. In isolated plastids the hydrolysis of the endogenous chlorophyll does not take place unless the membranes are solubilized in the presence of detergent. The structural latency of chlorophyllase activity appears to be due to the differential locations of substrate and enzyme within the plastids. Envelope membranes prepared from both chloroplasts and gerontoplasts contain chlorophyllase activity. The isolation of envelopes is associated with a marked increase in chlorophyllase activity per unit of protein. Yields of chlorophyllase and of specific envelope markers in the final preparations are similar, suggesting that the enzyme may be located in the envelope. It is hypothesized that the breakdown of chlorophyll during leaf senescence requires a mechanism that mediates the transfer of chlorophyll from the thylakoidal pigment-protein complexes to the sites of catabolic reactions in the envelope