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THE EVOLUTION OF THE PLACENTA and the implantation of a fetus changed the course of biology by permitting the emergence of mammals. Perhaps less known is the fact that this would never have occurred were it not for viral genes that got embedded into the genome early in the development of animals. View in LinkedIn

"ERVs belong to the class of retroelements possessing the three viral genes group-specific antigen (gag), polymerase (pol) and envelope (env) flanked by two long terminal repeats (LTR). Deletions and mutations resulted in inactivation of most of the 400,000 ERV copies, but some sustained an intact open reading frame and are able to express ERV sequences on RNA and protein level. In the majority of cases ERVs will be expressed tissue specific." https://lnkd.in/e8X7a46 View in LinkedIn

SPECIALIZED HERVs. "There is evidence that transcription of at least some HERV families may be differentially regulated depending on the cell type." http://jvi.asm.org/content/79/1/341.full View in LinkedIn

SILENCED. "Transcription of HERV-K and other HERV elements is usually suppressed by epigenetic factors such as DNA methylation and heterochromatin-silencing by histone modifications. During tumorigenesis (and also, presumably, in other poorly understood situations) this epigenetic control can be partially or selectively weakened and particular retroviral promoters become able to vigorously activate the transcription of cellular genes." http://journal.frontiersin.org/article/10.3389/fonc.2013.00246/full View in LinkedIn

FUNCTIONAL HERV LTRs. "The vast majority of these HERV elements are presumably not infectious or functional, but there are numerous reports on the expression of endogenous retroviral transcripts in human tissues and cell lines, mostly of placental, embryonic, or neoplastic origin. This suggests that a considerable number of HERV LTRs have retained their transcriptional activity." http://jvi.asm.org/content/79/1/341.full View in LinkedIn

200 HERV LTR FAMILIES. "Recent evaluation of the human genome sequencing data revealed that about 9% of the human genome is comprised of elements with long terminal repeats (LTRs) (LTR retrotransposons) comprising over 200 families." https://lnkd.in/bNEJmbG View in LinkedIn

CODING HERVS. "Nearly all HERVs known at present integrated up to 100 million years ago. They retain the proviral structure of exogenous retroviruses. Inherited from generation to generation, a few of the HERVs have open reading frames (ORFs) that encode known or putative proteins, while others are highly mutated." https://lnkd.in/ecX7YXX View in LinkedIn

OBLIGATE INTEGRATION. "Retroviruses are unique amongst animal viruses in that integration occurs as an obligate step in replication. For all other animal viruses integration occurs anomalously, through interaction with cellular retroelements such as LINEs, or via non-homologous recombination with genomic DNA." https://lnkd.in/bBjWGE5 View in LinkedIn

CLEAVAGE COMPLEXITY. "Not only are there differences in the number and placement of the cleavage sites, but also the recognition sites are relatively diverse, and many retroviral proteins contain sequences that would appear to be suitable sites for cleavage by the corresponding retroviral protease that remain uncleaved." https://lnkd.in/eBisbvi View in LinkedIn

SO ENDS this weekend on the subtle and ever-inventive molecular strategies of viruses and their hosts to act out their age-old relationships that range from benign to symbiotic to pathological. It is a glimpse of evolution's raw ingenuity engine, full of messy experiments that often lead to surprising outcomes. View in LinkedIn