Human Norovirus Histo-Blood Group Antigen (HBGA) Binding Sites Mediate the Virus Specific Interactions with Lettuce Carbohydrates

Lettuce is often implicated in human norovirus (HuNoV) foodborne outbreaks. We identified H-like histo-blood group antigens (HBGAs) on lettuce leaves as specific binding moieties for virus-like particles (VLPs) of HuNoV GII.4/HS194/2009 strain. The objective of this study was to determine whether HuNoV-lettuce binding is mediated through the virus HBGA binding sites (HBS). Toward this objective, VLPs of historical HuNoV GII.4 strains (1987, 1997, 2002, 2004 and 2006) with known natural mutations in their HBS, two newly generated VLP mutants of GII.4/HS194/2009 (D374A and G443A) and a VLP mutant (W375A) of GI.1/Norwalk/1968 along with its wild type VLPs, which displays distinct HBS, were investigated for their binding to lettuce. ELISA revealed that historical GII.4 strains binding to lettuce was dependent on their HBGAs profiles. The VLP mutants D374A and G443A lost binding to HBGAs and displayed no to minimal binding to lettuce, respectively. The VLPs of GI.1/Norwalk/1968 strain bound to lettuce through an H-like HBGA and the binding was inhibited by fucosidase digestion. Mutant W375A which was previously shown not to bind to HBGAs, displayed significantly reduced binding to lettuce. We conclude that the binding of HuNoV GII.4 and GI.1 strains to lettuce is mediated through the virus HBS.

Suppressor of sable [Su(s)] and Wdr82 down-regulate RNA from heat-shock-inducible repetitive elements by a mechanism that involves transcription termination

Although RNA polymerase II (Pol II) productively transcribes very long genes in vivo, transcription through extragenic sequences often terminates in the promoter-proximal region and the nascent RNA is degraded. Mechanisms that induce early termination and RNA degradation are not well understood in multicellular organisms. Here, we present evidence that the suppressor of sable [su(s)] regulatory pathway of Drosophila melanogaster plays a role in this process. We previously showed that Su(s) promotes exosome-mediated degradation of transcripts from endogenous repeated elements at an Hsp70 locus (Hsp70-αβ elements). In this report, we identify Wdr82 as a component of this process and show that it works with Su(s) to inhibit Pol II elongation through Hsp70-αβ elements. Furthermore, we show that the unstable transcripts produced during this process are polyadenylated at heterogeneous sites that lack canonical polyadenylation signals. We define two distinct regions that mediate this regulation. These results indicate that the Su(s) pathway promotes RNA degradation and transcription termination through a novel mechanism.

Drosophila suppressor of sable protein [Su(s)] promotes degradation of aberrant and transposon-derived RNAs

RNA-binding proteins act at various stages of gene expression to regulate and fine-tune patterns of mRNA accumulation. One protein in this class is Drosophila Su(s), a nuclear protein that has been previously shown to inhibit the accumulation of mutant transcripts by an unknown mechanism. Here, we have identified several additional RNAs that are downregulated by Su(s). These Su(s) targets include cryptic wild-type transcripts from the developmentally regulated Sgs4 and ng1 genes, noncoding RNAs derived from tandemly repeated alphabeta/alphagamma elements within an Hsp70 locus, and aberrant transcripts induced by Hsp70 promoter transgenes inserted at ectopic sites. We used the alphabeta RNAs to investigate the mechanism of Su(s) function and obtained evidence that these transcripts are degraded by the nuclear exosome and that Su(s) promotes this process. Furthermore, we showed that the RNA binding domains of Su(s) are important for this effect and mapped the sequences involved to a 267-nucleotide region of an alphabeta element. Taken together, these results suggest that Su(s) binds to certain nascent transcripts and stimulates their degradation by the nuclear exosome.

Suppressor of sable, a putative RNA-processing protein, functions at the level of transcription

The Drosophila melanogaster su(s) gene product negatively regulates the expression of mutant alleles with transposon insertions in the 5'-transcribed region by an unknown mechanism. We have investigated here su(s) function through in vivo structure-function analysis, heterologous reporter gene assays, and in vivo transcriptional induction experiments. We have shown that mutations of two arginine-rich motifs (ARMs), an acidic region, or two CCCH zinc fingers affect the ability of Su(s) to downregulate the expression of an insertion mutant allele and to autoregulate genomic su(s) transgenes. Using yeast and HeLa cell assays, we found that, when tethered to the promoter region, the N- and C-terminal regions of Su(s) can repress reporter gene expression, and all three motifs, but most significantly the ARMs, contribute to the repression activity. Finally, we showed that, in vivo, Su(s) inhibits the transcriptional induction of a transgene with an insertion in the first exon but does not affect induction of a similar transgene with a consensus 5' splice site near the upstream boundary of the insertion. Together, these results reveal a link between Su(s), transcription, and pre-mRNA processing.

Arginine-rich regions mediate the RNA binding and regulatory activities of the protein encoded by the Drosophila melanogaster suppressor of sable gene

The Drosophila melanogaster suppressor of sable gene, su(s), encodes a novel, 150-kDa nuclear RNA binding protein, SU(S), that negatively regulates RNA accumulation from mutant alleles of other genes that have transposon insertions in the 5' transcribed region. In this study, we delineated the RNA binding domain of SU(S) and evaluated its relevance to SU(S) function in vivo. As a result, we have defined two arginine-rich motifs (ARM1 and ARM2) that mediate the RNA binding activity of SU(S). ARM1 is required for in vitro high-affinity binding of SU(S) to small RNAs that were previously isolated by SELEX (binding site selection assay) and that contain a common consensus sequence. ARM1 is also required for the association of SU(S) with larval polytene chromosomes in vivo. ARM2 promotes binding of SU(S) to SELEX RNAs that lack the consensus sequence and apparently is neither necessary nor sufficient for the stable polytene chromosome association of SU(S). Use of the GAL4/UAS system to drive ectopic expression of su(s) cDNA transgenes revealed two previously unknown properties of SU(S). First, overexpression of SU(S) is lethal. Second, SU(S) negatively regulates expression of su(s) intronless cDNA transgenes, and the ARMs are required for this effect. Considering these and previous results, we propose that SU(S) binds to the 5' region of nascent transcripts and inhibits RNA production in a manner that can be overcome by splicing complex assembly.