Virology term 1 - negative ssRNA viruses Flashcards
Genes in -ive ssRNA viruses
RNA synthesis machinery
Fusion entry machinery
Capsid assembly machinery
Innate immunity antagonism
Size of -ssRNA viruses
Usually small; 4-12 genes.
-ssRNA genome features
No need to have functional elements of mRNAs - no cap or poly A tail.
Always needs protection, always bound to nucleocapsid.
Replication occurs via a separate +ssRNA species.
Mononegavirales
rhabdo, paramyxo, filo and borna.
Best known rhabdovirus
VSV, a cattle pathogen.
Rhabdovirus proteins
N - nucleocapsid P M G – spike protein. L - polymerase in nucleocapsid.
Paramyxovirinae family
Sendai,
Mumps,
Measles
Nipah
Pneumovirinae
RSV
Segmented -ssRNA viruses
- Arenaviridae - Lymphocytic choriomenigitis virus
- Bunyaviridae
- Tenuiviridae
- Orthmyxoviridae
Orthomyxoviridae polymerase
Polymerase is like L protein split into 3 bits
PA endonuclease activity
PB1 polymerase module
PB2 cap-binding activity
Conserved motifs in an Rdrp
7, A-F. Conservation usually homomorphs, chemically similar, rather than sequence identity.
Core domains of Rdrp
Thumb
Fingers
Palm
Thumb domain Rdrp
Involved in RNA binding, and in some it helps stabilize initiating NTPs. Thumb domain contributes to formation of NTP channel.
Finger domains in Rdrp
Finger subdomain residues pack into major groove of RNA template. I.e. important in template binding.
Palm domain Rdrp
Palm subdomain motifs are A, C and D. N-terminal aspartates co-ordinate two divalent metal ions critical for polymerase function. These co-ordinate NTPs. Motif D is important in co-ordinating binding of the correct NTP.
Additional domains to Rdrp - attachment
Flexibly attached by linker domains. Includes endonuclease domain.
Additional domains to Rdrp. E.g. in VSV and flaviviruses.
o Viral mRNa 5’ cap sysnthesis
o Methyltransferase
o Guanine-M7-methyltransferase
o Polyribonucleotidyl-transferase/guanylyltransferase.
Rdrp template recognition in
IAV: association with a promoter leads to conformations change in promoter directing transcriptional/replicative activity.
Flaviviridae: circularisation of genome and RNA structures in 5’ UTR
Picornaviridae: 5’ UTR structures and circularisation
Brome mosaic virus (+ssRNA) uses another protein to cause association.
Rdrp initiation - de novo
initiating nucleotide serves as primer for a second nucleotide. These are base paired with positions +1 and +2 of the template. This interaction needs stabilizing - usually done by residues within the Rdrp.
Rdrp structure if using primer dependent initiation.
Use of primed initiation means that a template channel which can accommodate several base pairs of dsRNA is needed, so thes polymerases lack palm and thumb domain protrusions. E.g. Coronaviridae and Picornaviridae Rdrps
Types of primer initiation for Rdrp
Oligonucleotide primed (cap-snatching), protein primed, back-primed.
Motion after initiation - Rdrp
requires conformational rearrangements from apo structure to open form. Binding of the correct nucleotide causes conformational changes leading to the formation of the closed complex (these conformational changes alter between viral families). After catalysis, reverts to open complex, with translocation of template-nascent strand duplex and release of PP¬i.
Rdrps fidelity
o Depends on conserved motifs of polymerase domain. Incorporation of nucleotides is relatively robust.
o No proof-reading domains mutation rate several orders of magnitude higher than for DdDps.
Topics to cover if ‘viral RNA and polypeptide synthesis’ asked
Transcription
Translation
Replication
Under these, consider initiation and control.
Mononegavirales - transcription
mRNAs need to be monocistronic. Single promoter: at each gene end sequence terminate mRNA and release, and either dissociate or reinitiate at next GS sequence.
Mononegavirales txn units
Use conserved gene start (GS) and gene end (GE) to give these.
Mononegavirales. The first GS signal.
Important: in RSV if this is mutated downstream sequences only transcribed at 10% of rate of WT.
Mononegavirales. mRNA release at gene end.
GE sequence and polyA tract.
Processivity through these may be intrinsic to Rdrp - mutations in RSV increase it specifically through GE tracts.
Mononegavirales. Scanning between GE and GS.
Highly efficient - even if tract is increased in length, RSV reinitiation can occur.
Mononegavirales: control of protein proportions.
Transcriptional; single pol entry point followed by attenuation (shown by UV target size study of Ball and White). Rhabdo.
Possibly partly because RNA associated with N, so cis-acting structures difficult to see, so has to interact with 3’ end and then travel along. Engages within Le promoter
Mononegavirales: control of protein proportions, gene order.
- VSV; gene order exerts strong control as polymerase has 20-30% chance of termination as traverses gene, and long pauses between genes.
Paramyxoviridae: control of protein production when polycistronic RNAs.
Sendai and measles for P(C/V).
Leaky scanning – Kozak and etc. Polymerase stuttering complicates.
P AUG most often used.
Sendai has 4 different ORFs accessed by different start sites for C. One is an ACG start codon. Variant P proteins, variant V proteins.
RNA pol stutter accesses cys rich V insertion.
Pneumoviridae: control of protein production when polycistronic RNAs.
Reinitiation translation for different proteins.
Also filoviruses.
Filoviruses: control of protein production when polycistronic RNAs.
RNA editing, reinitiation translation.
Viruses using splicing in nucleus.
Bornaviruses, orthomyxoviruses, filoviruses.
Influenza virus: transcription initiation.
capsnatching – takes cellular RNAs, degrades RNA and uses small amount of undegraded + cap to prime. PB1 causes cap-binding, PB2 cap-snatching. In RSV (paramyxo, pneumo) capping probably done by L protein.
Segmented -ssRNA sites - accessing multiple ORFs
Alternative translation initiation – N and NS in bunya.
Frameshifting for overlapping ORF (bunya)
Splicing – orthomyxoviruses.
Influenza virus: control of protein proportions.
Increased levels of early proteins needed to transcribe late protesin successfully
Increasing protein synthesis by host manipulation: -ssRNA viruses.
Decrease mRNA nuclear export.
Destroy mRNAs non-specifically.
Influenzavirus: decrease mRNA nuclear export.
Prevent cleavage of pre-mRNA…
Decrease polyadenylation by binding polyA polymerase binding protein.
Influenza: protein responsible for preventing pre-mRNA cleavage.
NS1 by binding the cellular polyadenylation/cleavage complex
Mononegavirales: replication vs transcription.
Use different promoters from transcriptase; replicase and transcriptase different complexes.
RSV replication promoters.
trailer at 3’ of genome contains promoter for replication. Complement to trailer at 5’ end leads to formation of sense genome from antisense cRNA.
Encapsidation of RSV
Cis acting sequence in Le RNA appears to promote encapsidation.
Controlling the switch between transcription and replication. Rhabdoviruses
Controlled by phosphorylation of P.
Seems to depend on 2 promotors (can only take place if enough newly synthesised N available for encapsidation of replicated genome —> N trans-acting regulation based on quantity - needed to form replicase)
Controlling the switch between transcription and replication. Paramyxoviruses.
promoter control over transcription vs replication
Controlling the switch between transcription and replication. Pneumoviridae.
control using M2 proteins; M2-1 is a an anti-terminator (to stop intragenic termination, but encourages processivity, acts by binding P), while M2-2 downregulates transcription and upregulates replication.
Controlling the switch between transcription and replication. RSV.
Needs Le sequence.
Packaging the right RNA. Paramyxo.
Imbalance of synthesis. Packaging of anti-genomes does occur.
Bipartite promoter associating N with 6 nucleotides.