No abstract

Although the best-defined function of type II major histocompatibility complex (MHC-II) is presentation of antigenic peptides to T lymphocytes, these molecules can also transduce signals leading alternatively to cell activation or apoptotic death. MHC-II is a heterodimer of two transmembrane proteins, each containing a short cytoplasmic tail that is dispensable for transduction of death signals. This suggests the function of an undefined MHC-II-associated transducer in signaling the death response. Here we describe a novel plasma membrane tetraspanner (MPYS) that is associated with MHC-II and mediates its transduction of death signals. MPYS is unusual among tetraspanners in containing an extended C-terminal cytoplasmic tail (ϳ140 amino acids) with multiple embedded signaling motifs. MPYS is tyrosine phosphorylated upon MHC-II aggregation and associates with inositol lipid and tyrosine phosphatases. Finally, MHC class II-mediated cell death signaling requires MPYS-dependent activation of the extracellular signal-regulated kinase signaling pathway.

Primers were designed to amplify a 592-bp region within a conserved structural gene (g20) found in some cyanophages. The goal was to use this gene as a proxy to infer genetic richness in natural cyanophage communities and to determine if sequences were more similar in similar environments. Gene products were amplified from samples from the Gulf of Mexico, the Arctic, Southern, and Northeast and Southeast Pacific Oceans, an Arctic cyanobacterial mat, a catfish production pond, lakes in Canada and Germany, and a depth of ca. 3,246 m in the Chuckchi Sea. Amplicons were separated by denaturing gradient gel electrophoresis, and selected bands were sequenced. Phylogenetic analysis revealed four previously unknown groups of g20 clusters, two of which were entirely found in freshwater. Also, sequences with >99% identities were recovered from environments that differed greatly in temperature and salinity. For example, nearly identical sequences were recovered from the Gulf of Mexico, the Southern Pacific Ocean, an Arctic freshwater cyanobacterial mat, and Lake Constance, Germany. These results imply that closely related hosts and the viruses infecting them are distributed widely across environments or that horizontal gene exchange occurs among phage communities from very different environments. Moreover, the amplification of g20 products from deep in the cyanobacterium-sparse Chuckchi Sea suggests that this primer set targets bacteriophages other than those infecting cyanobacteria.Unicellular cyanobacteria are a major component of the prokaryotic biomass in the oceans (6, 15), account for more than half of the fixed carbon in some regions, and play a key role in the transfer of energy through the microbial loop (6,15,21). Consequently, knowledge of the regulation of cyanobacterial communities is required to understand global nutrient and energy cycles.Viruses are the most abundant biological entities in fresh and marine waters and are typically 5-to 10-fold more abundant than prokaryotes (1,19,29). As mortality agents of heterotrophic and photosynthetic microbes (8,25,37), they affect the abundance and diversity of host cell communities (13, 18) as well as the cycling of carbon and nutrients (37).The proportion of primary producers lost to viral lysis is uncertain; however, viruses infecting a single strain of the marine cyanobacterium Synechococcus are widespread and can reach levels of abundance of Ͼ10 5 ml Ϫ1 (16,27,36). Estimates from the proportions of visibly infected cells and viral decay rates suggest that approximately 3 to 10% of Synechococcus spp. are destroyed daily by viral lysis (19,26,37).The impact of viruses on host communities can be inferred from viral community diversity, as viral taxonomy can provide insights into the hosts that they infect. For example, phylogenetic analyses of algal virus DNA polymerase genes revealed that phycoviruses are monophyletic relative to other viral families and that genetically distinct groups are clearly resolved and correspond to the host taxon infected (3, 4). Subsequ...

We examined diazotroph activity in controlled experiments containing natural populations of nitrogen (N 2 )-fixing microbes collected offshore from Kane'ohe Bay, Hawai'i, and from Station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the North Pacific subtropical gyre. Quantitative polymerase chain reaction and reverse transcription-polymerase chain reaction approaches showed that the abundances of the major phylotypes did not change substantially during experimental incubations. Diel phasing of nifH gene expression in the experiments was similar to that documented for natural populations. Cell-specific N 2 fixation rates, calculated as 15 N 2 incorporation rate divided by specific phylotype abundance (gene copy number), varied between the experiments, but generally agreed with the abundance and levels of nifH expression of each group of diazotrophs. Phosphorus additions had little effect on the abundance, gene expression, or N 2 fixation activity of diazotrophs across the course of the 36-h experiments, indicating that other factors may limit the growth, gene expression, and activity of diazotrophs in the North Pacific subtropical gyre.

To examine algal virus (Phycodnaviridae) genetic diversity in freshwater environments, gene fragments were cloned and sequenced from a river and a reservoir in Colorado, USA, and 2 different lakes in Ontario, Canada using PCR methods that target a diverse subset of known Phycodnaviridae DNA polymerase genes. Numerous phycodnavirus gene sequences were obtained from every sample, and rarefaction analysis of the sequence libraries demonstrated that virus richness was variable among different sample locations, and among samples collected from the same location at different times. Phylogenetic analysis of the unique sequences from each sample indicated that most sequences from the same geographic region (i.e. Colorado or Ontario) clustered together, but several exceptions were also observed. Phylogenetic analysis also demonstrated that the sequences obtained were more closely related to sequences from cultivated marine phycodnaviruses belonging to the genus Prasinovirus than to those from cultivated freshwater phycodnaviruses from the genus Chlorovirus. Overall, phycodnavirus sequences originating from cultivated marine viruses and marine clone libraries were not genetically distinct from the freshwater phycodnavirus sequences reported in this study.