A developmentally regulated gvpABC operon is involved in the formation of gas vesicles in the cyanobacterium Calothrix 7601

In the filamentous cyanobacterium Calothrix PCC7601, gas-vesicle (GV) formation is restricted to specialized filaments, called hormogonia. The differentiation of these cells is controlled by environmental factors, such as light intensity and/or wavelength. The structural gene (gvpA) encoding a GV protein in this cyanobacterium has been previously cloned and sequenced. Two other genes, gvpB and gvpC have been found in the sequence downstream from gvpA. The gvpB gene corresponds to a second copy of gvpA, encoding an identical protein. Unlike the GV protein, the product of the gvpC gene is predominantly hydrophilic, as deduced from nucleotide sequence. Interestingly, the internal part of the gvpC gene is composed of four contiguous repeats, each containing 99 bp, forming highly homologous repeats in the deduced amino acid sequence. Another kind of periodicity has been detected inside the 99-bp repeats, suggesting that the gvpC gene might have evolved by amplification of a 33-bp-long primordial building block. The function of this gene remains to be elucidated. Finally, we have shown that the three genes, gvpA, gvpB, and gvpC, are organized in an operon that is exclusively expressed during GV formation in hormogonia.     

Molecular cloning and nucleotide sequence of a developmentally regulated gene from the cyanobacterium Calothrix PCC 7601: a gas vesicle protein gene.

Since the gas vesicle protein (GVP) is highly conserved among the different gas-vacuolate prokaryotes, a 29-mer oligonucleotide corresponding to a portion of the Anabaena flos-aquae GVP gene was synthesized and used to isolate the GVP structural gene from Calothrix PCC 7601 (= Fremyella diplosiphon). Gas vacuole production in this filamentous cyanobacterium is restricted to hormogonia which occur at a specific stage during the developmental cell cycle. The GVP gene (gvpA) was localized on a 709 bp HindIII-HincII fragment. Nucleotide sequence analysis revealed a 213 bp open reading frame whose deduced amino-acid sequence shows a very high homology with that of the Anabaena flos-aquae GVP. Assuming that the first methionine residue is proteolytically processed, the molecular mass of the Calothrix GVP is 7375 daltons. Sequences resembling the Escherichia coli consensus promoter were found upstream from the gvpA gene. The initiator codon of the gvpA gene is preceded by a polypurine sequence assumed to be the ribosome binding site. Southern hybridizations with a probe specific for the gvpA gene indicated that this gene is not plasmid-borne, and that another homologous gene is present in the Calothrix genome.     

丝状体蓝藻藻殖段的分化及其调节机制

本文介绍了丝状体蓝藻(亦称蓝细菌) 的藻殖段的分化及其调节机制。藻殖段与正常藻丝体的区别在于细胞形状、细胞内存有气囊和可移动的短而直的藻丝链等。本文对许多环境因子包括光和营养因素等促进或抑制藻殖段的分化进行了讨论;还介绍了念珠藻(Nostoc) ,单歧藻(Tolypothrix) 和眉藻(Calothrix)所具有复杂的细胞发育过程,即具气囊又可移动的藻殖段分化,异形胞分化以及营养细胞的补偿性色适应。这三种细胞类型的适应形成取决于两种不同的光受体系统。藻殖段和异形胞两者的分化可能取决于光合电子传递链;而营养细胞的补偿性色适应则受光敏色素的调节。此外,谷酰胺合成酶合成和活性调节的PII蛋白,在协同藻殖段分化、异形胞分化及营养细胞的补偿色适应中起重要作用。由于蓝藻藻殖段分化及其调节机制是一个新的研究领域,关于它的知识尚不完整,亟待人们加强研究。     

Occurrence and distribution of gas vesicle genes among cyanobacteria.

Gas vesicles (GV) are specialized cell inclusions providing many aquatic procaryotes with buoyancy. In the cyanobacterium Calothrix sp. strain PCC 7601, at least four genes are involved in GV formation. One of those, gvpA1, encodes the major structural GV protein (70 amino acids) and belongs to a multigene family (gvpA1, gvpA2, gvpD). The fourth gene, gvpC, encodes a 162-amino-acid protein, the function of which is still unclear. We used the Calothrix gvpA1 and gvpC genes as probes to perform Southern hybridization experiments with DNA extracted from various cyanobacterial strains. The gvpA gene was found in all the strains that synthesize GV, indicating that its product is an obligatory component of GV. Furthermore, it was found to occur as multiple copies in most of the strains tested. The gvpC gene was only detected in some strains able to synthesize a large amount of GV within a short period. This suggests that the gvpC gene product is a dispensable protein for GV formation and is involved in the efficiency of the assembly process. Based on the occurrence of the gvp genes and on DNA-DNA hybridization patterns, genus assignments are discussed.     

丝状体蓝藻藻殖段的分化及其调节机制

本文介绍了丝状体蓝藻（亦称蓝细菌）的藻殖段的分化及其调节机制。藻殖段与正常藻丝体的区别在于细胞开状、细胞内存有气囊和可移动的短而真的藻丝链等。本文对许多环境因子包括光和营养因素等促进或抑制藻殖段的分化进行一讨论;还介绍了含球藻（Ｎｏｓｔｏｃ）,单歧藻（Ｔｏｌｙｐｏｔｈｒｉｘ）和眉藻（Ｃａｌｏｔｈｒｉｘ）所具有复杂的细胞发育过程,即具气囊又可移动的藻殖段分化,异形胞分化以及营养细胞的被偿性色适应。这     

Functional analysis of the gas vesicle gene cluster of the halophilic archaeon Haloferax mediterranei defines the vac-region boundary and suggests a regulatory role for the gvpD gene or its product

A series of deletions introduced into the gvp gene cluster of Haloferax mediterranei, comprising 14 genes involved in gas vesicle synthesis (mc-vac-region), was investigated by transformation experiments. Gas vesicle production and the expression of the gvpA gene encoding the major gas vesicle protein, GvpA, was monitored in each Haloferax volcanii transformant. Whereas transformants containing the entire mc-vac-region produced gas vesicles (Vac+), various deletions in the region 5' to gvpA (encompassing gvpD-gvpM) or 3' to gvpA (containing gvpC, gvpN and gvpO) revealed Vac- transformants. All these transformants expressed gvpA and contained the 8 kDa GvpA protein as shown by Western analysis. However, transformants containing the gvpA gene by itself indicated a lower level of GvpA than observed with each of the other transformants. None of these transformants containing deletion constructs assembled the GvpA protein into gas vesicles. In contrast, transformants containing a construct carrying a 918 bp deletion internal to gvpD exhibited a tremendous gas vesicle overproduction, suggesting a regulatory role for the gvpD gene or its product. This is the first assignment of a functional role for one of the 13 halobacterial gvp genes found in addition to gvpA that are involved in the synthesis of this unique structure.     

Electron Transport Regulates Cellular Differentiation in the Filamentous Cyanobacterium Calothrix

Differentiation of the filamentous cyanobacteria Calothrix sp strains PCC 7601 and PCC 7504 is regulated by light spectral quality. Vegetative filaments differentiate motile, gas-vacuolated hormogonia after transfer to fresh medium and incubation under red light. Hormogonia are transient and give rise to vegetative filaments, or to heterocystous filaments if fixed nitrogen is lacking. If incubated under green light after transfer to fresh medium, vegetative filaments do not differentiate hormogonia but may produce heterocysts directly, even in the presence of combined nitrogen. We used inhibitors of thylakoid electron transport (3-[3,4-dichlorophenyl]-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) to show that the opposing effects of red and green light on cell differentiation arise through differential excitations of photosystems I and II. Red light excitation of photosystem I oxidizes the plastoquinone pool, stimulating differentiation of hormogonia and inhibiting heterocyst differentiation. Conversely, net reduction of plastoquinone by green light excitation of photosystem II inhibits differentiation of hormogonia and stimulates heterocyst differentiation. This photoperception mechanism is distinct from the light regulation of complementary chromatic adaptation of phycobilisome constituents. Although complementary chromatic adaptation operates independently of the photocontrol of cellular differentiation, these two regulatory processes are linked, because the general expression of phycobiliprotein genes is transiently repressed during hormogonium differentiation. In addition, absorbance by phycobilisomes largely determines the light wavelengths that excite photosystem II, and thus the wavelengths that can imbalance electron transport.