A snippet of RNA from an intron in a gene can bind to an upstream regulatory element forming a triple helix and shut off transcription of the gene. Rather amazing don’t you think? Yet exactly was found in a far from obscure gene, the beta globin gene of hemoglobin on chromosome #11 [ Proc. Natl. Acad. Sci. vol. 116 pp. 6130 – 6139 ’19 ].
We’re talking large segments of DNA. There are five genes for the beta subunit of hemoglobin located from 5′ to 3′ as epsilon, gammaG, gammaA, delta and beta. The first 4 are expressed during fetal development. Beta globin is the one found in our red blood cells. The regulatory element controlling all 5 is found FIFTY kiloBases upstream from the beginning (5′ end) of beta globin.
The regulatory region is called the locus control region (LCR)and stretches over 20+ kiloBases. It has 7 sites where transcription factors bind (called hypersensitive sites HS1 — HS7). The hypersensitivity comes from the fact the chromosome is relative ‘open’ at these places and not compacted, so that an enzyme (DNAase I) can break the chromosome.
So after the beta globin gene is transcribed, the introns are spliced out, and the RNA from the second intron binds to HS2 forming a triple helix and displacing transcription factors bound there (USF2, GATA1, TAL1) which recruit RNA polymerase II (Pol II) In the normal course of events the whole mess would then march around the genome and eventually hit the promoter of beta globin (at least 50 kiloBases away) and turn on transcription.
This seems to be yet another mechanism of gene regulation. Just how widespread this is, isn’t known, but most protein coding genes have introns. Stay tuned.
Molecular biology is fascinating
Chemiotics II 