In the world of phage, uniqueness rules. The total number of phages in the biosphere is dramatic, with estimates numbering the population at 1031! Despite such high numbers, since the discovery of phage no identical phage has been found in the environment twice. They have often been termed "Nature's Most Successful Experiment." Their genomes are in constant flux amoung themselves and their hosts. It therefore should no longer surprise us when we discover a phage that doesn't quite fit the mold.In this paper by Pope, et al. they describe a phage (named Syn5) of the cyanobacteria that comes equipped with a "horn." Described as a "slender elongated fibrous protrusion," the horn clocks in at 50nm in length. To put perspective on this, the horn is almost as long as the capsid is wide (60nm) and is longer than the tail (25nm). It is however, quite thin 10nm wide at its thickest point and 2nm at its thinnest.
The figure below is a cryoEM taken of Syn5, showing the fibrous horn structure directly opposite the tail.
Every Syn5 capsid has this protrusion, and it only occurs once in its specific location.
The purpose is yet unknown, however there is a running hypothesis. These phages live out in the open ocean where there hosts are not around in very high numbers. Therefore, it would be highly beneficial for the phage to increase its chances of finding and attaching to a host. The phage would not want to be oriented in the wrong direction as a host cell passed overhead. If this horn functions as a host recognition site, clearly it would increase the phage's chances to find a host cell. To further amaze you, the vast majority of cyanophage carry their own photosynthesis genes!
I can only imagine what we will find next as we continue to explore this small world. Truly, All the World's a Phage.
Suggested Reading
The Bacteriophages
The Cyanobacteria: Molecular Biology, Genomics and Evolution
POPE, W., WEIGELE, P., CHANG, J., PEDULLA, M., FORD, M., HOUTZ, J., JIANG, W., CHIU, W., HATFULL, G., HENDRIX, R. (2007). Genome Sequence, Structural Proteins, and Capsid Organization of the Cyanophage Syn5: A “Horned†Bacteriophage of Marine Synechococcus. Journal of Molecular Biology, 368(4), 966-981. DOI: 10.1016/j.jmb.2007.02.046
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