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Hello, my name is Bryan Cullen.
I'm a professor in the Department of Molecular Genetics and
Microbiology at Duke University Medical Center in Durham, North Carolina.
The title of my talk today is viral epitranscriptomics.
What is epitranscriptomics?
The term is derived from the term epigenetics,
and is similar in that it refers to information
in the case of epitranscriptomics on RNA and
in the case of epigenetics on the DNA, that's not hardwired into the sequence of a gene.
If we look at viral and cellular mRNAs,
it's known that they are subject to what is called epitranscriptomic modification.
There are several different covalent modifications,
primarily methylations but also acetylations, of individual nucleotides in
eukaryotic mRNAs that have been detected at levels
greater than 0.05 percent of the parental nucleotide.
Of these, the most common is addition of a methyl group
to the N6 position of adenosine called m_6A,
but methylation of the C5 position of cytidine m_5C,
acetylation of the N4 position of cytidine,
ac4C, and methylation of the ribose moiety for all four bases
(this is called 2'-O-methyl modification
collectively abbreviated as Nm) are also prevalent.
The presence of high levels of m_6A in viral mRNAs was first
reported for influenza virus by Krug et al in 1976,
but no further progress was made on the significance of these modifications.
However, the modification was subsequently
identified in a range of viruses including retroviruses,
and as I said, the function of m_6A if anything remained unclear.
This schematic shows what has been uncovered about
m_6A addition over the last 10 years or so, by a number of different groups.
We know that there is a complex of
at least three proteins that adds m_6A to mRNAs as they're transcribed,
so it's a co-transcriptional process.
These m_6A groups can be recognized in
the nucleus by two proteins referred to as readers,
called YTHDC1 and YTHDC2,
but the more important readers are in fact in the cytoplasm and interact
with m_6A sites on mRNA after nuclear export of that mRNA.
These cytoplasmic readers are referred to YTHDF1, YTHDF2 and YTHDF3.
The YTHD part of their names refers
to the unit within the protein that actually binds to m_6A,
and that has been crystallized,
so it's clear that there's a very direct interaction between m_6A and the YTHDF domain.