Jeffrey D. DeMeis

Background/Objectives: Outer membrane vesicles (OMVs) are membrane-encapsulated spherical structures ~120 nm in diameter derived from Gram-negative bacterial cell envelopes. OMVs are primarily generated by outer membrane blebbing but contain proteins, DNA, and RNAs at concentrations distinct from that of the intracellular complement. OMVs have been associated with a number of different cellular functions including intercellular communication and resistance to phage. Methods: As bacterial small RNAs (sRNAs) also participate in bacteriophage defense and are specifically delivered to and enriched in OMVs, we recently elected to examine the effects of P22 infection on Salmonella cytosolic and OMV sRNA abundance by employing RNA sequencing. Results: We find that P22 infection triggers a global reduction in sRNAs (with Salmonella sRNA expression levels averaging only 15.6% those observed in noninfected cells) coupled with a reciprocal 72.7% global increase in Salmonella tRNA expression levels. Additionally, of note, while OMV small noncoding RNA (sncRNA) abundance is normally ~1/10 that found in the cytosol, we find that P22 infection triggers active OMV encapsulation and secretion of: (1) a subset of sRNAs, (2) all Salmonella tRNAs including one highly complementary to the P22 genome, and, much to our surprise, (3) ten distinct sRNAs expressed from P22. Conclusions: In summary, the work presented here identifies several Salmonella sncRNA cytosolic and/or OMV abundances significantly altered during P22 infection, and to our knowledge, this constitutes the first reported characterization of bacteriophage-encoded sRNAs being actively secreted within host OMVs.

Long G4-rich regions (LG4s) are defined as DNA sequences containing a high density of guanine triplets capable of forming non-B DNA structures called G-quadruplexes (G4s). These regions frequently overlap with enhancers, which are regulatory DNA elements that modulate gene expression by interacting with DNA regions that dictate where transcription is initiated known as promoters. While LG4s have now been well-characterized in the human genome, neither LG4 occurrence, nor the ability of LG4s to function as enhancers, in other species has been described. To address this, we screened the genomes of 16 different species from various taxa to identify LG4s and then determined if they were conserved, and if so, if their regulatory capacity was similarly conserved. Our analyses characterized a number of previously unreported LG4s in the human genome as well as LG4s in 13 additional species. Of note, we identified a highly conserved LG4 enhancer predicted to regulate over 40 genes. This LG4 is embedded in the MAZ (Myc-Associated Zinc finger protein) locus, and we find this LG4 possesses the ability to directly interact with the same target promoter in both human and mouse. In summary, this work describes LG4s in the genomes of both unicellular and multicellular species including vertebrates, invertebrates, plants, and fungi. Furthermore, many of these LG4 sequences are highly conserved as is their regulatory capacity.

Several studies have now described instances where G-rich sequences in promoters and enhancers regulate gene expression through forming G-quadruplex (G4) structures. Relatedly, our group recently identified 301 long genomic stretches significantly enriched for minimal G4 motifs (LG4s) in humans and found the majority of these overlap annotated enhancers, and furthermore, that the promoters regulated by these LG4 enhancers are similarly enriched with G4-capable sequences. While the generally accepted model for enhancer:promoter specificity maintains that interactions are dictated by enhancer- and promoter-bound transcriptional activator proteins, the current study tested an alternative hypothesis: that LG4 enhancers interact with cognate promoters via a direct G4:G4 DNA-based mechanism. This work establishes the nuclear proximity of LG4 enhancer:promoter pairs, biochemically demonstrates the ability of individual LG4 single-stranded DNAs (ssDNAs) to directly interact target promoter ssDNAs, and confirms that these interactions, as well as the ability of LG4 enhancers to activate target promoters in culture, are mediated by G4 DNA.

Enhancers are genomic sequences that function as regulatory elements capable of increasing the transcription of a given gene often located at a considerable distance. The broadly accepted model of enhancer activation involves bringing an enhancer-bound activator protein complex into close spatial proximity to its target promoter through chromatin looping. Equally relevant to the work described herein, roles for guanine (G) rich sequences in transcriptional regulation are now widely accepted. Non-coding G-rich sequences are commonly found in gene promoters and enhancers, and various studies have described specific instances where G-rich sequences regulate gene expression via their capacity to form G-quadruplex (G4) structures under physiological conditions. In light of this, our group previously performed a search for long human genomic stretches significantly enriched for minimal G4 motifs (referred to as LG4s herein) leading to the identification of 301 LG4 loci with a density of at least 80 GGG repeats / 1,000 basepairs (bp) and averaging 1,843 bp in length. Further, in agreement with previous reports indicating that minimal G4s are highly enriched in promoters and enhancers, we found 217/301 LG4 sequences overlap a GeneHancer annotated enhancer, and the gene promoters regulated by these LG4 enhancers were found to be similarly, markedly enriched with G4-capable sequences. Importantly, while the generally accepted model for enhancer:promoter specificity maintains that interactions are dictated by enhancer- and promoter-bound transcriptional activator proteins, the current study was designed to test an alternative hypothesis: that LG4 enhancers physically interact with their cognate promoters via a direct G4:G4 DNA-based mechanism. As such, this work employs a combination of informatic mining and locus-specific immunoprecipitation strategies to establish the spatial proximity of enhancer:promoter pairs within the nucleus then biochemically confirms the ability of individual LG4 ssDNAs to directly and specifically interact with DNA sequences found in their target promoters. In addition, we also identify four single nucleotide polymorphisms (SNPs), occurring within a LG4 enhancer on human chromosome 5, significantly associated with Cystic Fibrosis (CF) lung disease severity (avg. p value = 2.83E-9), presumably due to their effects on the expressions of CF-relevant genes directly regulated by this LG4 enhancer (e.g., EXOC3 and CEP72). Graphical AbstractIn brief: LG4 enhancers physically interact with gene promoters by forming composite G4 structures where both the LG4 and cognate promoter contribute half of the necessary sequence for G4 formation.