Male-specific single-stranded RNA (FRNA) coliphages belong to the family Leviviridae. They are classified into two genera (Levivirus and Allolevivirus), which can be subdivided into four genogroups (genogroups I and II in Levivirus and genogroups III and IV in Allolevivirus). Relatively few strains have been completely characterized, and hence, a detailed knowledge of this virus family is lacking. In this study, we sequenced and characterized the complete genomes of 19 FRNA strains (10 Levivirus strains and 9 Allolevivirus strains) and compared them to the 11 complete genome sequences available in GenBank. Nucleotide similarities among strains of Levivirus genogroups I and II were 75% to 99% and 83 to 94%, respectively, whereas similarities among strains of Allolevivirus genogroups III and IV ranged from 70 to 96% and 75 to 95%, respectively. Although genogroup I strain fr and genogroup III strains MX1 and M11 share only 70 to 78% sequence identity with strains in their respective genogroups, phylogenetic analyses of the complete genome and the individual genes suggest that strain fr should be grouped in Levivirus genogroup I and that the MX1 and M11 strains belong in Allolevivirus genogroup III. Strains within each genus share >50% sequence identity, whereas between the two genera, strains have <40% nucleotide sequence identity. Overall, amino acid composition, nucleotide similarities, and replicase catalytic domain location contributed to phylogenetic assignments. A conserved eight-nucleotide signature at the 3 end of the genome distinguishes leviviruses (5 ACCACCCA 3) from alloleviviruses (5 TCCTCCCA 3).

Significantly higher numbers of Gram-negative heterotrophic bacteria were present at the air-water interface (neston) of freshwater lakes than in the bulk water. Neuston bacteria were distinguished as a population distinct from bacteria in the bulk water by a higher incidence of pigmented colony types and significantly greater levels of multiple resistance to antibiotics and heavy metals. The incidence of plasmids in 236 neuston and 229 bulk water strains were similar (14 and 16.2%, respectively). Nine of 168 plasmid-free strains and 2 of 14 plasmid carrying strains, isolated from both bulk water and neuston, acted as recipients of plasmid R68.45 in plate matings with aPseudomonas aeruginosa donor strain PAO4032 at 21°C, but at frequencies below that of matings with a restriction-minus recipient strain ofP. aeruginosa, strain PAO1168. In a model system composed of nutrient-free synthetic lake water, plasmid R68.45 was shown to transfer betweenP. aeruginosa strains at frequencies between 10(-3) and 10(-5). Transconjugants were detected about 100 times more frequently at the interface than in the bulk water, which in part reflected a greater enrichment of the donor at this site. None of the aquatic isolates were able to act as recipients of plasmid R68.45 in this model system with strain PAO4032 as donor. The results suggest that under nutrient deprived conditions, the spread of plasmid R68.45 and similar plasmids by lateral transfer into this particular aquatic population would be a rare event.

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