From MicrobeWiki, the student-edited microbiology resource
A Microbial Biorealm page on the genus Gloeobacter violaceus
Higher order taxa
Bacteria; Cyanobacteria; Cyanophyceae; Chroococcales; Gloeobacteraceae
Description and significance
Gloeobacter violaceus is a rod-shape unicellular cyanobacterium that has been isolated from calcareous rocks in Switzerland . It is a Gram-negative, photoautotrophic, and aquatic cyanobacteria. The Gram stain test is useful in classifying two distinctly different types of bacteria based on structural differences in their cell walls. Gram-negative bacteria are those that do not retain crystal violet dye in the Gram staining protocol and Gram-positive bacteria will retain the dark blue dye after an alcohol wash. In a Gram stain test, a counterstain is added after the crystal violet, which colors all Gram-negative bacteria a red or pink color. Photoautotrophic means that Gloeobacter violaceus is an organism which derives energy from light and can manufacture their own food. Thus Gloeobacter violaceus is commonly labeled as a blue-green algae since it has the ability to live in the water and can manufacture it's own food. Although, it is small and unicellular, Gloeobacter violaceus tend to grow in colonies which are surrounded by a sticky mucous sac that plays a role in adhesion, thus these colonies are large enough to be visible with the human eye .
Gloeobacter violaceus is sensitive to strong light and a photoautotroph that contains chlorophyll a, carotenoids, and phycobiliproteins. Molecular phylogenetic analysis of Gloeobacter violaceus has revealed that this lineage has diverged from other cyanobacteria and also, that it possesses oxygenic photosynthesis characteristics. Another distinct quality of Gloeobacter violaceus is that its composition of fatty acids is different because it has a high content of polyunsaturated fatty acids (PUFA). A large content of PUFA is unusual for because it was thought that instead there should be a larger composition of (sulfoquinovosyl diacylglycerol SQDG) to ensure photosystem stabilization. These remarkable qualities make it important to sequence the genome of Gloeobacter violaceus because it will reveal the genetic background that is responsible for the origin and evolution of oxygenic photosynthesis. Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on earth. The primary step in this process is the conversion of sunlight into chemical energy and is driven by four, multisubunit, membrane-protein complexes that are known as photosystem I,photosystem II, cytochrome b6f and F-ATPase. The common method to determine the entire genome of Gloeobacter violaceus is the use of shotgun method in conjunction with the bridging shotgun strategy .
Scientist have revealed that the genome of Gloeobacter violaceus was comprised of a single circular chromosome that contained 4,659,019 base pairs. The average GC content was determined to be 62% and there was no detection of plasmids during the course of sequencing. The total number of potential protein encoding genes for the entire genome was determined to be 4430 or on average one gene for evey 1052 base pairs. Only 610 genes of Gloeobacter violaceus had matches to other cyanobacterial genomes, with about half of these having no known function. Also, about 20 % or 684 genes were found to be unique to this organism . Other interesting features of G. violaceus have been unveiled in comparisons of the assigned gene components with those of other cyanobacteria. It has revealed distinctive features of the G. violaceus genome, such as the genes for PsaI, PsaJ, PsaK, and PsaX for Photosystem I and PsbY, PsbZ and Psb27 for Photosystem II were missing, and those for PsaF, PsbO, PsbU, and PsbV were poorly conserved. CpcG for a rod core linker peptide for phycobilisomes and nblA related to the degradation of phycobilisomes were also missing. These observations can be explained due to the fact that photosynthesis in G. violaceus takes place not in thylakoid membranes but in the cytoplasmic membrane .
In a study by Tsuchiya et. al investigated the carotenoid composition and its biosynthetic pathway in the cyanobacterium Gloeobacter violaceus PCC 7421. They were able to identify two unique genes for carotenoid biosynthesis using in vivo functional complementation experiments. In Gloeobacter violaceus, a bacterial-type phytoene desaturase (CrtI), rather than plant-type desaturases (CrtP and CrtQ), produced lycopene. Lycopene is a red, fat-soluble pigment found in vegetables, and most commonly found in tomatoes. It is also one of a family of pigments called carotenoids. This study is one of the first demonstrations of an oxygenic photosynthetic organism utilizing bacterial-type phytoene desaturase. 
Cell structure and metabolism
Gloeobacter violaceus is a unicellular organism that exhibits atypical characteristics compared to other cyanobacteria. Its unusual characteristic is the lack of an internal thylakoid membrane system . Thylakoids are a membrane-bound compartment inside cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. The rod shaped phycobilisomes of this organism are heavily packed and attach to the plasmic surface of they cytoplasmic membrane. Phycobilisomes allow absorption and unidirectional transfer of light energy to chlorophyll a of the photosystem II. In this way, the cells take advantage of the available wavelengths of light (in the 500-650 nm range), which are inaccessible to chlorophyll, and uses tgat energy for photosynthesis. The cell walls of Gloeobacter violaceus are comprised of multiple layers. Starting from the exterior of the cell wall there is a sheath layer, then a double tracked structured outer membrane, intermediate cell wall layer that is electron dense, then lastly, a electron dense peptidoglycan layer. Peptidoglycan serves a structural role in the bacterial cell wall, it gives the wall shape and structural strength, as well as counteracting the osmotic pressure of the cytoplasm. Gloeobacter violaceus gets its energy from oxygenic photosynthesis which is unique to this organism because usually the machinery for photosynthesis is found in the thylakoid membrane and not in the cytoplasmic membrane. Important products that G. violaceus produces are petJ and petE which are used for soluble electron transfer catalysts. Also, Gloeobacter violaceus has a lower light requirement for growth than other cyanobacteria, but a higher than normal light saturation level for electron transfer rate . Electron transfer rate is the rate at which an electron moves from one atom or molecule to another atom or molecule.
The appearance of oxygenic photosynthetic organisms, such as Gloeobacter violaceus, on earth represents the largest discontinuous process in the evolution of photosynthetic organisms. Since the appearance of organisms that are oxygenic photosynthetic, scientist have revealed that they contribute the determining factors that has shifted the direction of global biological evolution through an increase in the oxygen concentration on the Earth . Oxygenic photosynthesis, which Gloeobacter violaceus performs is the conversion of sunlight into chemical energy.. It is extremely important in the survival of virtually all higher life forms. Also, the production of oxygen and the assimilation of carbon dioxide into organic matter determines, to a large extent, the composition of our atmosphere and provides all life forms with essential food and fuel. Another important characteristic of the cyanobacterium Gloeobacter violaceus, is that it contains Ths and GroEL and the recombination repair proteins RecA and RadA which enable it to exhibit characteristics that are similar to Archaea in expressing multiple detoxification PHX bacterioferritin comigratory proteins and several Hsp20s. Also, circadian rhythms have intensively been studied in cyanobacteria, and genes involved in various processes of circadian timing and regulation have been identified in many species of cyanobacteria. These genes include kaiABC as the major genetic elements of the circadian clock, sasA, cikA, ldpA, and pex as input modifiers, and rpoD2 and cpmA as output modifiers. Even after scientists completed an intensive search, they could not detect kaiABC in the Gloeobacter violaceus genome. It is therefore likely that Gloeobacter violaceus does not have a genetic controlling system for circadian rhythms and that cyanobacteria have acquired this system after divergence of the Gloeobacter violaceus lineage. Alternatively, Gloeobacter violaceus might have lost such genes . In general a circadian rhythm is an inherent cycle of approximately 24 hours in length that appears to control or initiate various biological processes. The natural signal for the circadian pattern is the change from darkness to light.
Gloeobacter violaceus and all species of the genus Gloeobacter are non-pathogenic thus they do not cause any known diseases to humans, animals, or plant hosts.
Application to Biotechnology
In cyanobacteria such as Gloebacter violaceus many compounds, including chlorophylls, carotenoids, and hopanoids, are synthesized from the isoprenoid precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate. Chlorophyll is the generic name for the intensely colored green pigments which are the photoreceptors of light energy in photosynthesis. Carotenoids are any of a class of yellow, orange, red, and purple pigments that are widely distributed in nature. They make it possible for photosynthetic organisms more fully to utilize the solar energy in the visible spectral region. Hopanoids are pentacyclic compounds similar to sterols, whose primary function is to improve plasma membrane fluidity in prokaryotes such as the cyanobacteria Gloeobacter violaceus. 
Also, a study completed by Koyama et al., revealed that two new linker proteins were identified by peptide mass fingerprinting in phycobilisomes isolated from the cyanobacterium Gloeobacter violaceus PCC 7421. The proteins were products of glr1262 and glr2806. Three tandem phycocyanin linker motifs similar to CpcC were present in each. The glr1262 product most probably functions as a rod linker connecting phycoerythrin and phycocyanin, while the glr2806 product may function as a rod-core linker. The observations in morphology of phycobilisomes in Gloeobacter violaceus is consistent with what has been reported about Gloeobacter violaceus's bundle-like shape with six rods. 
A study conducted by Steiger et al., 2005, revealed that Gloeobacter violaceus is a cyanobacterium isolated from other groups by lack of thylakoids and unique structural features of its photosynthetic protein complexes. They investigated carotenoid biosynthesis with respect to the carotenoids formed and the genes and enzymes involved. Their carotenoid analysis identified ss-carotene as major carotenoid and echinenone as a minor component. This composition is quite unique and the cellular amounts are up to 10-fold lower than in other unicellular cyanobacteria. Also, carotenoid biosynthesis was also found to be up-regulated in a light-dependent manner. This enhanced biosynthesis partially compensates for photooxidation especially of ss-carotene. They also sequenced the genome of Gloeobacter violaceus and analyzed several gene candidates homologous to carotenogenic genes from other organisms obtained. Functional expression of all candidates and complementation in Escherichia coli led to the identification of all genes involved in the biosynthesis of the G. violaceus carotenoids with the exception of the lycopene cyclase gene. An additional diketolase gene was found that functioned in E. coli but is silent in G. violaceus cells. The biggest difference from all other cyanobacteria is the existence of a single bacterial-type 4-step desaturase instead of the poly cis cyanobacterial desaturation pathway catalyzed by two cyanobacterial-type desaturases and an isomerase. The genes for these three enzymes are absent in G. violaceus .
A study performed by Krogmann et al., examined the complete genome sequence of Gloeobacter violaceus. This allowed them to understand better the structure of the phycobilisomes (PBS) of this cyanobacterium. In their study genomic analysis revealed peculiarities in these PBS: the presence of genes for two multidomain linker proteins, a core membrane linker with four repetitive sequences (REP domains), the absence of rod core linkers, two sets of phycocyanin (PC) alpha and beta subunits, two copies of a rod PC associated linker (CpcC), and two rod cap associated linkers (CpcD). They investigated the PBS proteins by gel electrophoresis, amino acid sequencing and peptide mass fingerprinting (PMF). They were able to conclude that two unique multidomain linkers contain three REP domains with high similarity and these were found to be in tandem and were separated by dissimilar Arms. One of these, with a mass of 81 kDa, is found in heavy PBS fragments rich in PC. They then proposed that it links six PC hexamers in two parallel rows in the rods. The other unique linker has a mass of 91 kDa and is easily released from the heavy fragments of PBS. We propose that this links the rods to the core. The presence of these multidomain linkers could explain the bundle shaped rods of the PBS. The presence of 4 REP domains in the core membrane linker protein (129 kDa) was established by PMF. This core linker may hold together 16 AP trimers of the pentacylindrical core, or alternatively, a tetracylindrical core of the PBS of G. violaceus.
Another study completed by Mimuro et al., they were able to conclude that since Gloeobacter violaceus clearly synthesizes MQ-4, their combined results indicate that this cyanobacterium must have a novel pathway for the synthesis of 1,4-dihydroxy-2-naphthoic acid. In their study they examined the secondary electron acceptor of photosystem (PS) I in the cyanobacterium Gloeobacter violaceus PCC 7421 and identified it as menaquinone-4 (MQ-4) by comparing high performance liquid chromatograms and absorption spectra with an authentic compound. The MQ-4 content was estimated to be two molecules per one molecule of chlorophyll (Chl) a', a constituent of P700. Comparative genomic analyses showed that six of eight men genes, encoding phylloquinone/MQ biosynthetic enzymes, are missing from the Gloeobacter violaceus genome.
More recently a study performed by Belkin et al., revealed that the internal pH values of two unicellular cyanobacterial strains could be determined with electron spin resonance probes, over an external pH range of 6 to 9, in the light and in the dark. In this study the slow growing, thylakoid-lacking Gloeobacter violaceus was found to have a low capacity for maintaining a constant internal pH. They also found that their was a distribution pattern of weak acid and amine nitroxide spin probes across the cell membranes of this organism, in the light and in the dark. This finding was consistent with the assumption that Gloeobacter violaceus contains a single intracellular compartment. 
Edited by Tracy Washington student of Rachel Larson and Kit Pogliano
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