Live cell microscope images show histones (green) at heat shock genes (indicated by the dotted lines), which are highly condensed under normal conditions (top). When the temperature of cells is raised, heat shock genes undergo extensive disruption (below) as seen by the formation of large puffs with RNA Polymerase II (red), as the histones unpack to make the gene available for expression. Image: Martin Buckley
The human genome contains some three billion base pairs that
are tightly compacted into the nucleus of each cell. If a DNA strand were the
thickness of a human hair, the entire human genome would be crammed into a
space the size of a softball, but if it were unraveled and all the strands
lined up, they would stretch from Ithaca, N.Y., to Boston.
University study, published
in Molecular Cell, teases out how cells undergo transcription, where
compacted DNA unravels, and a complex enzyme called RNA polymerase II reads the
desired gene’s DNA base pairs and transcribes them into RNA. The RNA then
instructs the cell to make specific proteins based on a gene’s blueprint.
In particular, the researchers expand on their previous work
that showed that the unraveling of compacted DNA occurs independently of
transcription by RNA polymerase II. Many scientists previously believed that
RNA polymerase II played a major role in decompacting the DNA.
“The process by which nucleosomes [coiled packets of
DNA] become decompacted—especially during transcription—hadn’t been well
characterized until the last decade,” said Steven Petesch, the paper’s
lead author and a graduate student in the laboratory of John Lis, the paper’s
senior author and the Barbara McClintock Professor of Molecular Biology and
Genetics at Cornell. “If you want to understand how basic processes like
transcription occur, then you need to understand the steps that facilitate that
process,” Petesch added.
To tease out such processes, Petesch and Lis used heat shock
genes from fruit flies, which become activated when temperatures rise above a
threshold, such as during hot days for fruit flies or when people get fevers.
The genes, which are found in many organisms, initiate processes that protect
cells from damage. By applying heat, the researchers were able to jump-start
within seconds the unfolding of DNA and the transcription of heat shock genes.
When the temperature rises, a protein called a heat shock
factor facilitates the steps necessary for transcription to occur. Among other
things, Petesch and Lis found that heat shock factor binds to the heat shock
genes and activates processes involving key enzymes that ultimately cause the
enzyme Poly(ADP-ribose) polymerase (PARP) to locally produce Poly(ADP-Ribose)
(PAR), a long polymer similar to DNA and RNA.
When DNA is compacted, strands wrap tightly around proteins
called histones, like thread wrapped around a spool, to create packets of
nucleosomes. But, it turns out, PAR competes with DNA for binding of histone
proteins, which helps them unwrap from their spools and decompact. The
researchers found that within seconds of increasing temperatures, heat shock
factor is recruited to start the process of modifying histones and the
unfolding of the DNA before transcription occurs.
“This process is completely independent of RNA
Polymerase II transcribing through the gene,” said Petesch. Lis added,
“However, this process is completely dependent on heat shock factor and
PARP’s ability to make PAR chains, which are necessary to rapidly remove
nucleosomes and unravel the induced gene.”
PARP has been known for decades as important for DNA repair
and is targeted in some cancer treatments.
SOURCE – Cornell University