Mini viral RNAs act as innate immune agonists during influenza virus infection

We thank the High-Throughput Genomics Group at the Wellcome Trust Centre for Human Genetics (funded by Wellcome Trust grant 090532/Z/09/Z) for the generation of adapter-ligated mvRNA sequencing data. This work was supported by the Wellcome Trust grant 098721/Z/12/Z, the joint Wellcome Trust and Royal Society grant 206579/Z/17/Z and a Netherlands Organization for Scientific Research (NWO) grant 825.11.029 to A.J.W.t.V.; EPA Cephalosporin Junior Research Fellowship to D.L.V.B.; support by the Intramural Research Program of NIAID, NIH, to E.d.W.; Research Grants Council of the Hong Kong Special Administrative Region, China, project no. T11-705/14N and a Croucher Senior Research Fellowship to L.L.M.P.; and Medical Research Council (MRC) programme grants MR/K000241/1 and MR/R009945/1 to E.F. and studentship to J.C.L.The molecular processes that determine the outcome of influenza virus infection in humans are multifactorial and involve a complex interplay between host, viral and bacterial factors1. However, it is generally accepted that a strong innate immune dysregulation known as ‘cytokine storm’ contributes to the pathology of infections with the 1918 H1N1 pandemic or the highly pathogenic avian influenza viruses of the H5N1 subtype2,3,4. The RNA sensor retinoic acid-inducible gene I (RIG-I) plays an important role in sensing viral infection and initiating a signalling cascade that leads to interferon expression5. Here, we show that short aberrant RNAs (mini viral RNAs (mvRNAs)), produced by the viral RNA polymerase during the replication of the viral RNA genome, bind to and activate RIG-I and lead to the expression of interferon-β. We find that erroneous polymerase activity, dysregulation of viral RNA replication or the presence of avian-specific amino acids underlie mvRNA generation and cytokine expression in mammalian cells. By deep sequencing RNA samples from the lungs of ferrets infected with influenza viruses, we show that mvRNAs are generated during infection in vivo. We propose that mvRNAs act as the main agonists of RIG-I during influenza virus infection.PostprintPeer reviewe

Preparing and sequencing ultra-long DNA molecules from single chromosomes

In this thesis, I describe the development of a single-molecule platform for analysing long DNA molecules that captures haplotype and large-scale structural variation (SV) in addition to DNA sequence. Current DNA sequencing methods cannot adequately examine haplotype and SV – both contribute to biological function and disease and are candidates for the location of "missing heritability" in the genome. Both haplotype and SV fundamentally relate to the structure of single chromosomes. Using a lab-on-a-chip nanofluidic device, SV was analysed on stretched (&amp;GT; 2 Mb) DNA fragments. In order to integrate this larger-scale SV information with the base-by-base sequence of the molecule being analysed, the DNA molecule was amplified and sequenced. I developed algorithms to handle the unique features of sequence data from amplified single DNA molecules. I obtained sequence and genotyping data, confirmed successful isolation of single DNA fragments from the chip, and validated the barcoding method used to detect SV. This lab-on-a-chip device for handling long DNAs can also serve as a 'reaction chamber' to answer more fundamental biological questions regarding chromosome structure as a whole. Using a microfluidic chip, I was able to provide the first direct images of DNA catenation within metaphase chromosomes and demonstrate that DNA catenation, in addition to proteins, plays a crucial role in metaphase chromosome architecture. The fluidic platform can be adapted to future 'third-generation' single-molecule sequencing applications that interrogate single DNA molecules directly. I have demonstrated this potential in two ways: First, I used intercalating dyes to form an optical waveguide along DNA to improve single-molecule detection. Secondly, I engineered E. coli RNA Polymerase to detect single base translocation events along a DNA substrate. Such a polymerase could be used in future third-generation sequencing schemes based upon base-stepping motion or energy transfer to dye-modified nucleotides as the polymerase processes on a long DNA template.</p

Real-Time Single-Molecule Studies of RNA Polymerase–Promoter Open Complex Formation Reveal Substantial Heterogeneity Along the Promoter-Opening Pathway

The expression of most bacterial genes commences with the binding of RNA polymerase (RNAP)–σ70 holoenzyme to the promoter DNA. This initial RNAP–promoter closed complex undergoes a series of conformational changes, including the formation of a transcription bubble on the promoter and the loading of template DNA strand into the RNAP active site; these changes lead to the catalytically active open complex (RPO) state. Recent cryo-electron microscopy studies have provided detailed structural insight on the RPO and putative intermediates on its formation pathway. Here, we employ single-molecule fluorescence microscopy to interrogate the conformational dynamics and reaction kinetics during real-time RPO formation on a consensus lac promoter. We find that the promoter opening may proceed rapidly from the closed to open conformation in a single apparent step, or may instead involve a significant intermediate between these states. The formed RPO complexes are also different with respect to their transcription bubble stability. The RNAP cleft loops, and especially the β′ rudder, stabilise the transcription bubble. The RNAP interactions with the promoter upstream sequence (beyond −35) stimulate transcription bubble nucleation and tune the reaction path towards stable forms of the RPO.</p

Real-Time Single-Molecule Studies of RNA Polymerase–Promoter Open Complex Formation Reveal Substantial Heterogeneity Along the Promoter-Opening Pathway

The expression of most bacterial genes commences with the binding of RNA polymerase (RNAP)–σ70 holoenzyme to the promoter DNA. This initial RNAP–promoter closed complex undergoes a series of conformational changes, including the formation of a transcription bubble on the promoter and the loading of template DNA strand into the RNAP active site; these changes lead to the catalytically active open complex (RPO) state. Recent cryo-electron microscopy studies have provided detailed structural insight on the RPO and putative intermediates on its formation pathway. Here, we employ single-molecule fluorescence microscopy to interrogate the conformational dynamics and reaction kinetics during real-time RPO formation on a consensus lac promoter. We find that the promoter opening may proceed rapidly from the closed to open conformation in a single apparent step, or may instead involve a significant intermediate between these states. The formed RPO complexes are also different with respect to their transcription bubble stability. The RNAP cleft loops, and especially the β′ rudder, stabilise the transcription bubble. The RNAP interactions with the promoter upstream sequence (beyond −35) stimulate transcription bubble nucleation and tune the reaction path towards stable forms of the RPO

Fully Streched Single DNA Molecules in a Nanofluidic Chip Show Large-Scale Structural Variation

When stretching and imaging DNA molecules in nanofluidic devices, it is important to know the relation between the physical length as measured in the lab and the distance along the contour of the DNA. Here a single DNA molecule longer than 1 Mbp is loaded into a nanofluidic device consisting of two crossing nanoslits (85nm x 50 microns) connected to microchannels. An applied pressure creates a stagnation point at the crossing of the nanoslits. The drag force from the fluid stretches the DNA. We determine the degree of stretching of the molecule (i) without the use of markers, (ii) without knowing the contour length of the DNA, and (iii) without having the full DNA molecule inside the field-of-view. The analysis is based on the transverse motion of the DNA due its Brownian motion, i.e. the DNA's response to the thermal fluctuations of the liquid surrounding it. The parameter values obtained by fitting agree well with values we obtain from simplified modeling of the DNA as a cylinder in a parallel flow. Secondly, DNA molecules stained with the intercalating dye YOYO-1 are de- and renatured locally following a modified version of the protocol used in Ref. 1. The result is a melting pattern which reflects the local AT/GC-content. Single molecules are loaded into the chip and imaged. Due to the almost complete stretching of the DNA, structural variations in the size range from kbp to Mbp can be detected and quantified from the melting pattern alone

Pausing controls branching between productive and non-productive pathways during initial transcription in bacteria

Transcription in bacteria is controlled by multiple molecular mechanisms that precisely regulate gene expression. It has been recently shown that initial RNA synthesis by the bacterial RNA polymerase (RNAP) is interrupted by pauses; however, the pausing determinants and the relationship of pausing with productive and abortive RNA synthesis remain poorly understood. Using single-molecule FRET and biochemical analysis, here we show that the pause encountered by RNAP after the synthesis of a 6-nt RNA (ITC6) renders the promoter escape strongly dependent on the NTP concentration. Mechanistically, the paused ITC6 acts as a checkpoint that directs RNAP to one of three competing pathways: productive transcription, abortive RNA release, or a new unscrunching/scrunching pathway. The cyclic unscrunching/scrunching of the promoter generates a long-lived, RNA-bound paused state; the abortive RNA release and DNA unscrunching are thus not as tightly linked as previously thought. Finally, our new model couples the pausing with the abortive and productive outcomes of initial transcription.BN/Martin Depken La