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In cyanobacteria, light energy is mainly harvested by the phycobiliproteins that form the phycobilisome rods, and funneled to the photosynthetic reaction centers through the core components. Among them, allophycocyanin (alpha AP, beta AP) and the so-called LCM play a major role. This report deals with the characterization of the apcE gene from Synechococcus sp. PCC 6301 which specifies the LCM. It maps upstream from the apcA gene (alpha AP). Transcriptional analyses demonstrate that the apcABC gene cluster (alpha AP, beta AP, and LC7.8) forms an operon, whereas the apcE gene behaves as a monocistronic unit. The functional organization of the apcEABC gene cluster, as well as of the apcE gene product, of Synechococcus 6301 are compared to their counterparts in three other organisms. Finally, a model is proposed for the architecture of the phycobilisome core.
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PMID:The "anchor polypeptide" of cyanobacterial phycobilisomes. Molecular characterization of the Synechococcus sp. PCC 6301 apce gene. 190 65

Cyanobacteria harvest light energy through multimolecular structures, the phycobilisomes, regularly arrayed at the surface of the photosynthetic membranes. Phycobilisomes consist of a central core from which rods radiate. A large polypeptide (LCM, 75-120 kDa) is postulated to act both as terminal energy acceptor and as a linker polypeptide that stabilizes the phycobilisome architecture. We report here the characterization of the gene (apcE) that encodes this LCM polypeptide in Calothrix sp. PCC 7601. It is located upstream from the genes encoding the major components of the phycobilisome core (allophycocyanin) and is part of the same operon. The deduced amino acid sequence shows that the N-terminal region of LCM shares homology with the other phycobiliprotein subunits and thus constitutes the chromoprotein domain. The other part of the molecule is made up of four repeated domains that are highly homologous to the N-terminal regions of the phycocyanin rod linker polypeptides. The predicted secondary structure of the different domains of the LCM is discussed in relation to the different roles and properties of this large molecule.
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PMID:Molecular characterization of the terminal energy acceptor of cyanobacterial phycobilisomes. 210 46

In cyanobacteria, phycobilisomes are regularly arrayed on the surface of the photosynthetic membranes, and their role is to funnel light energy to the underlying photosystem II reaction center. A model has recently been proposed that ascribes to the so-called LCM, a central role in the building up of the phycobilisome, in addition to its role of terminal energy acceptor (Capuano, V., Braux, A.-S., Tandeau de Marsac, N., and Houmard, J. (1991) J. Biol. Chem. 266, 7239-7247). The phycobilisomes of Calothrix sp. PCC 7601 are typically of the type found in most cyanobacteria. Those of Synechococcus PCC 7942 (or PCC 6301) differ in having central cores made up of two instead of three cylinders. We have integrated the Calothrix PCC 7601 apcE gene that encodes the LCM into the chromosome of a Synechococcus PCC 7942 strain. We have observed that the heterologous gene is expressed and that the corresponding product carries a bilin-type chromophore and can be detected in the phycobilisome fraction of the Synechococcus strain. Moreover, it is shown that, in agreement with our model, this LCM can direct the formation of phycobilisomes that have three-cylinder cores.
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PMID:An in vivo approach to define the role of the LCM, the key polypeptide of cyanobacterial phycobilisomes. 768 41

Genes encoding the phycobilisome core subunits allophycocyanin alpha and beta and a small core linker protein in Synechocystis sp. strain PCC 6714 were cloned and sequenced. These genes form an operon, apcABC, with a single transcription start site and two possible termination sites, one following apcB and the other following apcC. The promoter region, like those of the apcABC operons of other cyanobacteria, does not resemble the consensus promoter sequences of Escherichia coli. However, the apcABC promoters identified in four strains of cyanobacteria have conserved sequences centered at -50 and -10 with respect to the start of transcription. The apcE gene, encoding the protein that links the phycobilisome core to the thylakoid membrane, was also cloned from Synechocystis 6714 and sequenced. It is unlinked to the apcABC operon. As in other Synechocystis strains, the LCM polypeptide encoded by the apcE gene contains three repeats of the basic phycobiliprotein linker domain. The apcE gene promoter sequence bears little resemblance to either the E. coli consensus or the apcABC promoter region, but it is similar to the corresponding regions of other cyanobacterial apcE genes. In these cases, there are conserved sequences centered at -40 and -10 with respect to the transcription start site. These conserved promoter elements from the apcABC and apcE genes were also identified in the corresponding 5'-flanking regions of eleven transcript starts for cpc genes encoding phycocyanin subunits in cyanobacteria and algal chloroplasts. These results suggest that a factor yet to be described participates in transcription of phycobiliprotein genes.
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PMID:Isolation and characterization of the genes encoding allophycocyanin subunits and two linker proteins from Synechocystis 6714. 846 79

The effect of UVB irradiation on the phycobilisomes (PBSs) of Synechococcus sp. PCC 7942 cells was studied. The sucrose density-gradient-isolated PBSs from in vivo UVB-treated (280-320 nm) cells showed a strong decrease in beta-phycocyanin (beta PC) and alpha-phycocyanin (alpha PC) polypeptides, in addition to a decrease in the linker polypeptides LCM 75 (linker connecting the core to the thylakoid membranes), LR 33 (linker in the rod structure), LRC 31.5 (linker connecting the rod to the core) and LRC 29. In vitro UVB treatment of gradient-isolated intact PBSs for 1 h had no effect on any of the constituent polypeptides, and only after 2 h was a degradation of LCM 75 and LR 33 and a decrease in beta PC evident. Further investigation of phycobiliproteins (4 h of UVB irradiation) using polyclonal antibody directed against purified whole PBSs revealed that, in vivo, there was a gradual decline in the levels of LCM 75, LR 33, LRC 31.5, LRC 29, beta PC and alpha PC.
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PMID:UVB-induced photodamage to phycobilisomes of Synechococcus sp. PCC 7942. 937 11

The phycobilisome (PBS) of Anabaena sp. PCC 7120 was allowed to dissociate into its constituents and the resulting allophycocyanin (AP) fraction was purified. Its reconstitution yielded a complex which according to negative stain electron microscopy and spectral analysis was identical to the native pentacylindrical PBS core domain. Each cylinder of the central tricylindric unit was comprised of four AP (alphabeta)3 disks. Mass analysis using the scanning transmission electron microscope (STEM) showed the presence of 16 AP trimers in the intact reconstitute, which had a total mass of 1966(+/-66) kDa. Composition analysis indicated an AP trimer distribution of (AP-II):(AP-LCM):(AP-B):(AP-I)=6:2:2:6, i.e. an addition of two AP-I and two AP-II complexes compared to a tricylindrical PBS core domain. Therefore, we suggest that each supplementary half-core cylinder found in pentacylindrical AP core domains is comprised of one AP-I and one AP-II trimer, in agreement with the current model. The structural significance of the 127 kDa core membrane linker polypeptide was further investigated by subjecting the AP core reconstitute to mild chymotryptic degradation. After isolation, the digested complex exhibited a tricylindrical appearance while STEM mass analysis confirmed the presence of only 12 AP complexes. Polypeptide analysis by SDS-PAGE and Edman degradation related the half-cylinder loss to cleavage of the Rep4 domain of the core membrane linker polypeptide. On the basis of these data, a general model for the assembly of the three hemidiscoidal PBS types known to date is discussed.
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PMID:Reconstitution, characterisation and mass analysis of the pentacylindrical allophycocyanin core complex from the cyanobacterium Anabaena sp. PCC 7120. 957 Oct 58

The core-membrane linker, LCM, connects functionally the extramembraneous light-harvesting complex of cyanobacteria, the phycobilisome, to the chlorophyll-containing core-complexes in the photosynthetic membrane. Genes coding for the apoprotein, ApcE, from Nostoc sp. PCC 7120 and for a C-terminally truncated fragment ApcE(1-240) containing the chromophore binding cysteine-195 were overexpressed in Escherichia coli. Both bind covalently phycocyanobilin (PCB) in an autocatalytic reaction, in the presence of 4M urea necessary to solubilize the proteins. If judged from the intense, red-shifted absorption and fluorescence, both products have the features of the native core-membrane linker LCM, demonstrating that the lyase function, the dimerization motif, and the capacity to extremely red-shift the chromophore are all contained in the N-terminal phycobilin domain of ApcE. The red-shift is, however, not the result of excitonic interactions: Although the chromoprotein dimerizes, the circular dichroism shows no indication of excitonic coupling. The lack of homologies with the autocatalytically chromophorylating phytochromes, as well as with the heterodimeric cysteine-alpha84 lyases, indicates that ApcE constitutes a third type of bilin:biliprotein lyase.
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PMID:Reconstitution of phycobilisome core-membrane linker, LCM, by autocatalytic chromophore binding to ApcE. 1562 Mar 67