A cyanophage, PaV-LD, has been isolated from harmful filamentous cyanobacterium Planktothrix agardhii in Lake Donghu, a shallow freshwater lake in China. Here, we present the cyanophage's genomic organization and major structural proteins. The genome is a 95,299-bp-long, linear double-stranded DNA and contains 142 potential genes. BLAST searches revealed 29 proteins of known function in cyanophages, cyanobacteria, or bacteria. Thirteen major structural proteins ranging in size from 27 kDa to 172 kDa were identified by SDS-PAGE and mass-spectrometric analysis. The genome lacks major genes that are necessary to the tail structure, and the tailless PaV-LD has been confirmed by an electron microscopy comparison with other tail cyanophages and phages. Phylogenetic analysis of the major capsid proteins also reveals an independent branch of PaV-LD that is quite different from other known tail cyanophages and phages. Moreover, the unique genome carries a nonbleaching protein A (NblA) gene (open reading frame [ORF] 022L), which is present in all phycobilisome-containing organisms and mediates phycobilisome degradation. Western blot detection confirmed that 022L was expressed after PaV-LD infection in the host filamentous cyanobacterium. In addition, its appearance was companied by a significant decline of phycocyanobilin content and a color change of the cyanobacterial cells from blue-green to yellow-green. The biological function of PaV-LD nblA was further confirmed by expression in a model cyanobacterium via an integration platform, by spectroscopic analysis and electron microscopy observation. The data indicate that PaV-LD is an exceptional cyanophage of filamentous cyanobacteria, and this novel cyanophage will also provide us with a new vision of the cyanophage-host interactions. Cyanophages are one kind of planktonic viruses that infect cyanobacteria (blue-green algae). Cyanophages and phages have amazing amounts of genetic diversity and biological activity in water environments (33,34,40,54). In either a freshwater or saltwater environment, cyanophages are ubiquitous and play an important role in water ecosystems (46,52,61,66). Generally, the complete genome sequences of cyanophages can provide significant clues for better understanding of the biological properties, ecological effects, and coevolutionary relationships between cyanophages and their hosts (10,17,18,21,27,29). Some cyanophage genomes have been sequenced (32,35,44,49,51,60,64), which has revealed the presence of cyanobacterial genes involved in central energy metabolism and their host's survival. For examples, some photosynthesis-related genes (psbA, hliP, and PSII) and stress-response genes (coding for chaperones and genes associated with bacterial motility and chemotaxis) have been described in cyanophages (4, 5, 31, 36, 47), most of which are transcribed together with essential cyanophage replication-related genes (6, 13, 30, 65). Moreover, a nonbleaching protein A (NblA) gene has been found from a lytic phage, Ma-LMM01, infecting Microcystis aerugi...
Cyanophages, a group of viruses specifically infecting cyanobacteria, are genetically diverse and extensively abundant in water environments. As a result of selective pressure, cyanophages often acquire a range of metabolic genes from host genomes. The host-derived genes make a significant contribution to the ecological success of cyanophages. In this review, we summarize the host-derived metabolic genes, as well as their origin and roles in cyanophage evolution and important host metabolic pathways, such as the light-dependent reactions of photosynthesis, the pentose phosphate pathway, nutrient acquisition and nucleotide biosynthesis. We also discuss the suitability of the host-derived metabolic genes as potential diagnostic markers for the detection of genetic diversity of cyanophages in natural environments.
A previously unknown cyanophage, PaV-LD (Planktothrix agardhii Virus isolated from Lake Donghu), which causes lysis of the bloom-forming filamentous cyanobacterium P. agardhii, was isolated from Lake Donghu, Wuhan, China. PaV-LD only lysed P. agardhii strains isolated from Lake Donghu and not those isolated from other lakes. The PaV-LD particle has an icosahedral, non-tailed structure, ca. 70 to 85 nm (mean ± SD = 76 ± 6 nm) in diameter. PaV-LD was stable at freezing temperature, but lost its infectivity at temperatures > 50°C. Lysis of host cells was delayed about 3 d after the PaV-LD treatment with chloroform, and the virus was inactivated by exposure to low pH (≤4). The latent period and burst size of the PaV-LD were estimated to be 48 to 72 h and about 340 infectious units per cell, respectively. The regrowth cultures of surviving host filaments were not lysed by the PaV-LD suspension. To our knowledge, this is the first isolation and cultivation of a virus infectious to the filamentous bloom-forming cyanobacterium Planktothrix from a freshwater lake.
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