On 2017 Jul 17, Donald Forsdyke commented:

IF G-QUADRUPLEXES, WHY SO MANY ADENINES?

It is good to see the problem of EBV immune evasion focused, not on the translation product of EBNA1 mRNA (1), but on the mRNA itself (2). However, it is puzzling that the sequence encoding the glycine-alanine repeats is enriched not only in guanines (Gs), but also in adenines (As). In such a GC-rich genome (60% GC), there is a scarcity of As, yet they are concentrated in the glycine-alanine repeat-encoding region. In other words, codons have been selected for their general purine-richness, not just for their G-richness (3). While it is conceivable that the As somehow assist consecutive Gs to form G-quadruplexes, consideration might have been given to the hypothesis that the G-quadruplexes may merely be helpful by-products of the fundamental need to purine-load the mRNA.

EBV is not alone in this respect. EBV and HTLV-1 share common characters. Both are deeply latent, GC-rich viruses. They persist in their human hosts for long periods often with no obvious detrimental effects. Most of their proteins are encoded by pyrimidine-rich mRNAs. The HTLV-1 provirus encodes its pyrimidine-rich mRNAs in its "top" sense strand. But there is a "bottom" strand transcript. This is heavily R-loaded and is translated into a basic zipper protein (HBZ) which is poorly immunogenic and is increasingly seen, like EBNA-1, as playing a major role in immune evasion (4-6).

1. 1.Levitskaya, J. et al. (1995) Inhibition of antigen processing by the internal repeat region of the Epstein-Barr virus nuclear antigen-1. Nature 375:685–688. Levitskaya J, 1995
2. 2.Lista MJ et al. (2017) Nucleolin directly mediates Epstein-Barr virus immune evasion through binding to G-quadruplexes of EBNA-1 mRNA. Nature Commun 8:16043. Lista MJ, 2017
3. 3.Cristillo AD et al. (2001) Double-stranded RNA as a not-self alarm signal: to evade, most viruses purine-load their RNAs, but some (HTLV-1, Epstein-Barr) pyrimidine-load. J Theor Biol 208:475–491.Cristillo AD, 2001
4. 4.Cook LB et al. (2013) HTLV-1: Persistence and pathogenesis. Virology 435:131–140. Cook LB, 2013
5. 5.Shiohama et al. (2016) Absolute quantification of HTLV-1 basic leucine zipper factor (HBZ) protein and its plasma antibody in HTLV-1 infected individuals with different clinical status. Retrovirology 13:29 Shiohama Y, 2016
6. 6.Forsdyke DR EBV Webpage

On 2017 Jul 17, Donald Forsdyke commented:

IF G-QUADRUPLEXES, WHY SO MANY ADENINES?

It is good to see the problem of EBV immune evasion focused, not on the translation product of EBNA1 mRNA (1), but on the mRNA itself (2). However, it is puzzling that the sequence encoding the glycine-alanine repeats is enriched not only in guanines (Gs), but also in adenines (As). In such a GC-rich genome (60% GC), there is a scarcity of As, yet they are concentrated in the glycine-alanine repeat-encoding region. In other words, codons have been selected for their general purine-richness, not just for their G-richness (3). While it is conceivable that the As somehow assist consecutive Gs to form G-quadruplexes, consideration might have been given to the hypothesis that the G-quadruplexes may merely be helpful by-products of the fundamental need to purine-load the mRNA.

EBV is not alone in this respect. EBV and HTLV-1 share common characters. Both are deeply latent, GC-rich viruses. They persist in their human hosts for long periods often with no obvious detrimental effects. Most of their proteins are encoded by pyrimidine-rich mRNAs. The HTLV-1 provirus encodes its pyrimidine-rich mRNAs in its "top" sense strand. But there is a "bottom" strand transcript. This is heavily R-loaded and is translated into a basic zipper protein (HBZ) which is poorly immunogenic and is increasingly seen, like EBNA-1, as playing a major role in immune evasion (4-6).

1. 1.Levitskaya, J. et al. (1995) Inhibition of antigen processing by the internal repeat region of the Epstein-Barr virus nuclear antigen-1. Nature 375:685–688. Levitskaya J, 1995
2. 2.Lista MJ et al. (2017) Nucleolin directly mediates Epstein-Barr virus immune evasion through binding to G-quadruplexes of EBNA-1 mRNA. Nature Commun 8:16043. Lista MJ, 2017
3. 3.Cristillo AD et al. (2001) Double-stranded RNA as a not-self alarm signal: to evade, most viruses purine-load their RNAs, but some (HTLV-1, Epstein-Barr) pyrimidine-load. J Theor Biol 208:475–491.Cristillo AD, 2001
4. 4.Cook LB et al. (2013) HTLV-1: Persistence and pathogenesis. Virology 435:131–140. Cook LB, 2013
5. 5.Shiohama et al. (2016) Absolute quantification of HTLV-1 basic leucine zipper factor (HBZ) protein and its plasma antibody in HTLV-1 infected individuals with different clinical status. Retrovirology 13:29 Shiohama Y, 2016
6. 6.Forsdyke DR EBV Webpage