Junk that isn’t

The more we understand, the more we realize how little we’ve understood what we thought we understood.   Here is a double example.

We have 1,400,000 Alu elements in our genome.  They are about 300 nucleotides long, meaning that there is over 1 every 3,000 nucleotides in our 3,200,000,000 nucleotide genome.  They don’t code for protein, and were widely thought to be junk, selfish genes whose only role was to ensure that the organism carrying them, kept them along as they reproduced.

This post contains a heavy dose of contemporary molecular biology.  If you’re a little shaky on some of it have a look at — https://luysii.wordpress.com/2010/07/07/molecular-biology-survival-guide-for-chemists-i-dna-and-protein-coding-gene-structure/ — and follow the links.

Not so says Proc. Natl. Acad. Sci. vol. 117 pp. 415 – 425  ’20.  They are part of several important physiologic processes (1) T lymphocyte activation (2) heat shock stress (3) endoplasmic reticulum stress.  All 3 cause transcription of Alu’s by RNA polymerase III (pol III).

All RNA levels increase with heat shock, including RNAs made from Alu elements.  They bind directly and tightly (nanoMolar affinity) to RNA polymerase II (which transcribes protein coding genes) and co-occupy the promoters of repressed genes, preventing transcription of these genes and protein synthesis of them.  At least that was the state of play 11 years ago (PNAS 105 5569 – 5574 ’09)

This paper notes that Alu is not passive, but actually a self-cleaving ribozyme (an enzyme made of RNA), an entirely new role.  When complexed with another protein EZH2 (a polycomb protein thought to be a transcriptional repressor using its lysine methylation activity), the rate of Alu self-cleavage increases by 40%.

So what?

In addition to stoping transcription, Alu also retards transcription elongation.  So stress increases in EZH2 causes Alu to cleave itself faster, turning off  repression and improving the responses to the 3 types of stresses above.

So we really didn’t understand both Alu which has been studied for years, or EZH2 a polycomb protein (ditto).  Alu is a self-cleaving ribozyme, and EZH2 doesn’t just turn off genes by its enzymatic activity (lysine trimethylation), but binds to an RNA so it can cleave itself faster (e.g. its a cofactor).

Fascinating and humbling to see how much there is to know about things we thought we knew.  But it’s also exciting.  Who knows what else is out there to discover about the known, never mind the known unknowns.

Yet another mechanism of gene regulation

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

The twists and turns of topoisomerase (pun intended)

It is very sad that my late friend Nick Cozzarelli isn’t around to enjoy the latest exploits of the enzyme class he did so much great work on — the topoisomerases. For a social note about him see the end of the post.

We tend to be quite glib about just what goes on inside a nucleus when DNA is opened up and transcribed into mRNA by RNA polymerase II (Pol II). We think of DNA has a linear sequence of 4 different elements (which it is) and stop there. But DNA is a double helix, and the two strands of the helix wind around each other every 10 elements (nucleotides), meaning that within the confines of our nuclei this happens 320,000,000 times.

I’ve written a series of six posts on what we would see if our nuclei were enlarged  by a factor of 100,000 (which is the amount of compaction our DNA must undergo to fit inside the 10 micron (10 millionths of a meter) in diameter nucleus (since if fully extended our DNA would be 1 meter long. So if you compacted the distance from New York to Seattle (2840 miles or 14,995,200 feet) down by this factor you’d get a sphere 150 feet in diameter or half the length of a football (US) field. Now imagine blowing up the diameter and length of the DNA helix by 100,000 and you’d get something looking like a 2,840 mil long strand of linguini which twists on itself  320,000,000 times. The two strands are 3/8th of an inch thick. They twist around each other every 9/16ths of an inch.

For the gory details start at https://luysii.wordpress.com/2010/03/22/the-cell-nucleus-and-its-dna-on-a-human-scale-i/ and follow the links.

Well, we know that for DNA to be copied into mRNA it must be untwisted, the strands separated so RNA polymerase II (Pol II) can get to it.  Pol II is enormous — a mass of 500 kiloDaltons and 7 times thicker at 140 Angstroms than the DNA helix of 20 Angstrom thickness.

Consider the fos gene (which we’ll be talking about later). It contains 380 amino acids (meaning that the gene contains at least 1140 nucleotides ). The actual gene is longer because of introns (3,461 nucleotides), which means that the gene contains 346 complete turns of the double helix, all of which must be unwound to transcribe it into mRNA.

So it’s time for an experiment. Get about 3 feet of cord roughly 3/8 of an inch thick. Tie the ends together, loop one end around a hook in your closet, put a pencil in the other end and rotate it about 100 times (or until you get tired). Keeping everything the same, have a friend put another pencil between the two strands in the middle, separating them. Now pull on the strands to make the separation wider and move the middle pencil toward one end. In the direction of motion the stands will coil even tighter (supercoiling) and behind they’ll unwind.

This should make it harder for Pol II to do its work (or for enzymes which copy DNA to more DNA). This is where the various topoisomerase come in. They cut DNA allowing supercoils to unwind. They remain attached to the DNA they cut so that the DNA can be put back together. There are basically two classes of topoisomerase — Type I topoisomerase cuts one strand, leaving the other intact, type II cuts both.

Who would have thought that type II topoisomerase would be involved in the day to day function of our brain.

Neurons are extended things, with information flowing from dendrites on one side of the cell body to much longer axons on the other. The flow involves depolarization of the cell body as impulses travel toward the axon. We know that certain genes are turned on by this activity (e.g. the DNA coding for the protein is transcribed into mRNA which is translated into protein by the ribosome). They are called activity dependent genes.

This is where [ Cell vol. 1496 – 1498, 1592 – 1605 ’15 ] comes in. Prior to neuronal activity, when activity dependent genes are expressed at low levels, the genes still show the hallmarks of highly expressed genes (e.g. binding by transcription factors and RNA polymerase II, Histone H3 trimethylation of lysine #4 {H3K4Me3 } at promoters).

This work shows that such genes are highly negatively supercoiled (see above) preventing RNA polymerase II (Pol II) from extending into the gene body. On depolarization of the cell body in some way Topoisomerase IIB is activated, leading to double strand breaks (dsbs) within promoters allowing the DNA to unwind and Pol II to productively elongate through gene bodies.

There is evidence that neuronal stimulation leads to dsbs ( Nature NeuroScience vol. 16 pp. 613 – 621 ’13 ) throughout the transcription of immediate early genes (e.g. genes turned on by neural activity). The evidence is that there is phosphorylation of serine #139 on histone variant H2AX (gammaH2AX) which is a chromatin mark deposited on adjacent histones by the DNA damage response pathway immediately after DSBs are found.

Etoposide (a topoisomerase inhibitor) traps the enzyme in a state where it remains bound to the DNA of the dsb. On etoposide Rx, there is an increase in activity dependent genes (Fos, FosB, Npas4). Inhibition of topiosomerase IIB (the most prevalent topoisomerase in neurons) by RNA interference (RNAi) leads to blunted activity dependent induction of these genes. This implies that DNA cutting by topoisomerase IIB is required for gene activation in response to neuronal activity.  Other evidence is that knocking down topoisomerase  using RNA interference (RNAi) stops activity dependent gene transcription.

Further supporting this idea, the authors induced dsbs at promoters of activity dependent genes (Fos, fosB, Npas4) using the CRISPR system. A significant increase in transcription was found when the Fos promoter was targeted.

I frankly find this incredible. Double strand breaks are considered bad things for good reason and the cell mounts huge redundant machines to repair them, yet apparently neurons, the longest lived cells in our bodies are doing this day in and day out. The work is so fantastic that it needs to be replicated.

Social Note: Nick Cozzarelli is one of the reasons Princeton was such a great institution back in the 50s (and hopefully still is). Nick’s father was an immigrant shoemaker living in Jersey City, N. J. Princeton recognized his talent, took him in, allowing him to work his way through on scholarship, waiting tables in commons, etc. etc. He obtained a PhD in biochemistry from Harvard and later became a prof at Berkeley, where he edited the Proceedings of the National Academy of Sciences USA for 10 years. He passed away far too soon of Burkitt’s lymphoma in his late 60s. We were friends as undergraduates and in grad school.

I can only wonder what Nick would say about the latest twists of the topoisomerase story