A Novel Freshwater Cyanophage, Mae-Yong924-1, Reveals a New Family

Abstract

Cyanobacterial blooms are a worldwide ecological issue. Cyanophages are aquatic viruses specifically infecting cyanobacteria. Little is known about freshwater cyanophages. In this study, a freshwater cyanophage, Mae-Yong924-1, was isolated by the double-layer agar plate method using Microcystis aeruginosa FACHB-924 as an indicator host. Mae-Yong924-1 has several unusual characteristics: a unique shape, cross-taxonomic order infectivity and a very unique genome sequence. Mae-Yong924-1 contains a nearly spherical head of about 100 nm in diameter. The tail or tail-like structure (approximately 40 nm in length) is like the tassel of a round Chinese lantern. It could lyse six diverse cyanobacteria strains across three orders including Chroococcales, Nostocales and Oscillatoriales. The genome of the cyanophage is 40,325 bp in length, with a G + C content of 48.32%, and 59 predicted open reading frames (ORFs), only 12 (20%) of which were functionally annotated. Both BLASTn and BLASTx scanning resulted in "No significant similarity found", i.e., the Mae-Yong924-1 genome shared extremely low homology with sequences in NCBI databases. Mae-Yong924-1 formed a root node alone and monopolized a root branch in the proteomic tree based on genome-wide sequence similarities. The results suggest that Mae-Yong924-1 may reveal a new unknown family apparently distinct from other viruses.

Keywords: Microcystis aeruginosa; Microcystis virus; complete genome; freshwater phage; new clade.

Figure 1

Morphology of cyanophage Mae-Yong924-1. Transmission electron micrograph of negatively stained phage particle. Scale bar, 100 nm.

Figure 2

Macro- and micrographs of M. elabens FACHB-916 cultures. (A) Macrograph of normal FACHB-916 cultures; (B) Macrograph of FACHB-916 co-inoculated with cyanophage Mae-Yong924-1; (C) Micrograph of normal FACHB-916 cultures; (D) Micrograph of FACHB-916 co-inoculated with cyanophage Mae-Yong924-1.

Figure 3

Genome map of cyanophage Mae-Yong924-1. The outermost circle represents the 59 ORFs encoded in the genome, with different colors representing different functions (clockwise arrow indicates the forward reading frame, and counterclockwise arrow indicates the reverse reading frame); the dark circles in the middle represent the GC content (outwards indicates greater than the average GC content compared with the whole genome, and inwards indicates the opposite); the innermost circle represents the GC skew (G − C/G + C; outwards indicates >0, and inwards indicates <0).

Figure 4

Proteomic tree based on the complete genome sequences of Mae-Yong924-1 (red star), 10 other cyanophages (black star) including 5 Microcystis cyanophages, 43 representative bacteriophages of Caudovirales and 3 viruses of Herpesvirales.