Di- and trimethylated congeners of 7-methylguanine in Sindbis virus mRNA

Box H/ACA snoRNAs are preferred substrates for the trimethylguanosine synthase in the divergent unicellular eukaryote Trichomonas vaginalis

“…This finding of DMG caps in the parasite strain T1 harboring the dsRNA virus is in accordance with previous observations that some eukaryal RNA virus transcripts have DMG caps (HsuChen and Dubin 1976;van Duijn et al 1986). In addition, the same enzymatic limitation of mimivirus Tgs in vitro leads to a speculative scenario in which mimivirus would synthesize DMG-capped viral mRNAs to compete for the host translation machinery.…”

Trimethylguanosine capping selectively promotes expression of Rev-dependent HIV-1 RNAs

“…Compatible with our results, in Caenorhabditis elegans and Ascaris lumbricoides, nematode mRNAs can acquire TMG caps by transsplicing, and in these settings, virtually all actin and ribosomal protein mRNAs are TMG-capped and loaded onto polysomes and are functionally translated with efficiency (59)(60)(61). Similarly, during the replication of togaviruses (e.g., Semliki Forest virus and Sindbis virus), late viral mRNAs acquire hypermethylated guanosine caps, and the expression of late proteins is proficient (21,22).…”

“…It has been generally assumed that like cellular RNAs, many viral RNAs have a 5′ m7G cap, and that viruses either use the cellular capping machinery (e.g., viruses that replicate in the nucleus) or encode their own capping enzymes (e.g., viruses that replicate in the cytoplasm) (15,16). Interestingly, there is a surprisingly diverse range of cap modifications; for instance, Mimivirus, a large DNA virus of amoeba, encodes an RNA cap guanine-N2 methyltransferase that hypermethylates the viral RNA cap (17); influenza virus "snatches" caps from cellular mRNAs (18,19); West Nile fever virus has two methyl additions on its RNA cap (20); Sindbis virus produces mRNAs that have dimethylguanosine and trimethylguanosine caps [hypermethylated caps/m 2,2,7 G caps (21)]; and Semliki forest virus late mRNAs also have hypermethylated caps (22). On the other hand, poliovirus, encephalomyelitis virus, foot and mouth disease virus, and Calici virus are examples of viral RNAs lacking 5′ caps (11,23), and there is limited in vitro evidence that HIV-1 RNAs are capped (24,25).…”

Characterization of a mimivirus RNA cap guanine-N2 methyltransferase

“…Because it is not yet feasible to genetically engineer mimivirus, the role of MimiTgs in the virus replication cycle cannot be evaluated. However, we can speculate on the potential role of DMG caps in viral mRNAs given that: (1) they are known to be present in certain RNA virus transcripts (HsuChen and Dubin 1976;Van Duijn et al 1986); (2) DMG caps confer an advantage in mRNA translation vis a vis an otherwise identical reporter mRNA containing a standard m 7 G cap (Darzynkiewicz et al 1988;Cai et al 1999);and (3) this advantage is a specific feature of a DMG cap, i.e., a TMG cap renders mRNA severalfold less translatable than a standard m 7 G-capped transcript (Darzynkiewicz et al 1988;Cai et al 1999). An appealing scenario is that mimivirus chooses to synthesize DMG-modified capped mRNAs in order to achieve preferential translation of viral polypeptides versus those of the host cell, Acanthamoeba polyphaga.…”

“…A 2,2,7-trimethylguanosine (TMG) cap is found on noncoding eukaryal RNAs such as small nuclear (sn) and small nucleolar (sno) RNAs and telomerase RNA (Busch et al 1982;Seto et al 1999) and on nematode mRNAs that undergo trans-splicing of a 59-capped leader sequence (Liou and Blumenthal 1990). A 2,7-dimethylguanosine (DMG) cap is present in the mRNAs of two RNA viruses: Sindbis virus and Semliki Forest virus (HsuChen and Dubin 1976;Van Duijn et al 1986). Whereas TMG synthesis has attracted interest because of the involvement of snRNAs in pre-mRNA splicing, there has been little attention paid to potential roles for DMG and TMG caps in virus biology since such structures were described in Sindbis virus mRNAs more than 30 years ago.…”