Genomic and Seasonal Variations among Aquatic Phages Infecting the Baltic Sea Gammaproteobacterium Rheinheimera sp. Strain BAL341.

Knowledge in aquatic virology has been greatly improved by culture-independent methods, yet there is still a critical need for isolating novel phages to identify the large proportion of "unknowns" that dominate metagenomes and for detailed analyses of phage-host interactions. Here, 54 phages infecting Rheinheimera sp. strain BAL341 (Gammaproteobacteria) were isolated from Baltic Sea seawater and characterized through genome content analysis and comparative genomics. The phages showed a myovirus-like morphology and belonged to a novel genus, for which we propose the name Barbavirus All phages had similar genome sizes and numbers of genes (80 to 84 kb; 134 to 145 genes), and based on average nucleotide identity and genome BLAST distance phylogeny, the phages were divided into five species. The phages possessed several genes involved in metabolic processes and host signaling, such as genes encoding ribonucleotide reductase and thymidylate synthase, phoH, and mazG One species had additional metabolic genes involved in pyridine nucleotide salvage, possibly providing a fitness advantage by further increasing the phages' replication efficiency. Recruitment of viral metagenomic reads (25 Baltic Sea viral metagenomes from 2012 to 2015) to the phage genomes showed pronounced seasonal variations, with increased relative abundances of barba phages in August and September synchronized with peaks in host abundances, as shown by 16S rRNA gene amplicon sequencing. Overall, this study provides detailed information regarding genetic diversity, phage-host interactions, and temporal dynamics of an ecologically important aquatic phage-host system.IMPORTANCE Phages are important in aquatic ecosystems as they influence their microbial hosts through lysis, gene transfer, transcriptional regulation, and expression of phage metabolic genes. Still, there is limited knowledge of how phages interact with their hosts, especially at fine scales. Here, a Rheinheimera phage-host system constituting highly similar phages infecting one host strain is presented. This relatively limited diversity has previously been seen only when smaller numbers of phages have been isolated and points toward ecological constraints affecting the Rheinheimera phage diversity. The variation of metabolic genes among the species points toward various fitness advantages, opening up possibilities for future hypothesis testing. Phage-host dynamics monitored over several years point toward recurring "kill-the-winner" oscillations and an ecological niche fulfilled by this system in the Baltic Sea. Identifying and quantifying ecological dynamics of such phage-host model systems in situ allow us to understand and study the influence of phages on aquatic ecosystems.

Regulation of neuronal nitric oxide synthase by histone, protamine, and myelin basic protein.

We examined the effects of endogenous basic proteins rich in the amino acid L-arginine on neuronal NO synthase activity by monitoring cyclic GMP formation in intact neuron-like neuroblastoma N1E-115 cells. Histone, protamine and myelin basic protein significantly stimulated cyclic GMP formation, both in a time- and concentration-dependent manner. These effects were blocked by hemoglobin and NO synthase inhibitors. Removal of the extracellular/intracellular Ca2+ gradient by a Ca2+ chelator completely abolished the cyclic GMP responses elicited by histone and protamine, suggesting that influx of extracellular Ca2+ might be involved in their activation of NO synthase. The effects of myelin basic protein on cyclic GMP formation, however, appeared to be due to Ca2+ release from intracellular stores. In cytosolic preparations of rat cerebellum, these basic proteins inhibited the metabolism of L-arginine into L-citrulline by NO synthase. We conclude from our findings that endogenous basic proteins might be involved in the regulation of neuronal NO synthase activity. Their effects on the enzyme could be either stimulatory or inhibitory, depending on whether the basic proteins exert their effects extracellularly or intracellularly, respectively.