Cyanophages are one kind of planktonic viruses that infect cyanobacteria (blue-green algae). Cyanophages have amazing amounts of genetic diversity and biological activity in water environments. They play important roles not only in the regulation of the population density and structure of cyanobacteria, but also in the biogeochemical cycle. Due to its potential application in the biological control of harmful cyanobacterial blooms, cyanophage has become a hot issue in aquatic virology and water environmental research.  

Recently, the genomic organization and biological function of nblA gene of the first tailless cyanophage, PaV-LD, were clarified together by two research groups lead by Profs. ZHANG Qiya and GUI Jianfang at Institute of Hydrobiology, Chinese Academy of Sciences (IHB). PaV-LD was isolated from Lake Donghu, a shallow freshwater lake in Wuhan, China, and could infect the ？lamentous cyanobacterium Planktothrix agardhii. The genome is a 95,299-bp-long, linear double-stranded DNA and contains 142 putative genes.  

Thirteen major structural proteins ranging in size from 27kDa to 172kDa 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 in 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 ？lamentous cyanobacterium. In addition, its appearance was accompanied 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 electronmicroscopy observation. The data indicate that PaV-LD is an exceptional cyanophage of ？lamentous cyanobacteria, and this novel cyanophage will also provide us with a new vision of understanding the genetic diversity of cyanophages and cyanophage-host interactions. 

The research was fulfilled by IHB Ph. D student GAO Ebin et al. under the supervision of Prof. ZHANG Qiya. This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (KSCX2-EW-Z-3), the National Natural Science Foundation of China (30871938, 31072239) and the National Major Basic Reaearch Program (2010CB126303, 2009CB118704). The results have been published online in Journal of Virology (http://jvi.asm.org/content/86/1/236).