Field of Science

Storing DNA

ResearchBlogging.orgDNA is one of the most important components of the cell. In eukaryote cells (i.e the cells of humans and plants) it is stored inside a nucleus that keeps it safe and away from dangerous things like free radicals produced by the metabolic reactions of the cell. In bacterial cells the DNA isn't nearly as well protected, but the main bulk of the bacterial chromosome (excluding the little floating plasmids) is all kept together in a bundle usually referred to as a nucleoid.

However the DNA in cells is rather long which means that in order to package it into a small space it needs to be coiled up. Eukaryotic cells do this by using proteins called Histones, which coil the DNA around them, and can also signal which genes the cell should be turning into proteins.

Image from wikimedia

Because eukaryotes cells have so much DNA they need to keep it all tightly coiled up, with little molecular tags on the histones to remind them which bits are needed reguarly, and which bits can be mostly ignored. Prokaryotes on the the other hand don't need quite as much control, most of their DNA is going to be used most of the time, but it still needs to be packaged up to fit inside the cell.

The cells do this using by using the unbelieveably eukaryot-ist named "histone-like proteins" such as H-NS which is found in E. coli and related bacteria. Although their precise mechanisms are not quite clear, they have been shown to play a clear role in the coiling of the DNA and maintaining nuceolide stability. They also (like their eukaryote counterpart) help to control gene expression as well as chromatin structure.

In most cases, the H-NS functions as a transcription repressor merely through its physical presence wrapped around DNA - it's harder to transcribe DNA (the first step for protein production) when it's all wrapped up tightly in balls. Some genes though, seem to be activated when associated with H-NS, as it can affect the stability of the mRNA transcript. These are usually genes associated with stress responses; in fact many of the genes that are affected by H-NS are linked to the stress response or changes in the environmental conditions, such as high or low temperature, high osmolarity, changes in pH or oxygen concentration.

Quite how the H-NS controls these parts of the DNA is not entirely clear. Unlike histones, which are positively charged to stabilise the negative charge on the DNA, H-NS proteins are neutral. Although the domain of the molecule that binds to the DNA has been identified, it is not yet certain which parts of these are vital, or how they interact with the DNA molecule. One thing that is clear however is that H-NS are capible of forming dimers, and probably carry out much of their task as a dimer of two joined molecules.

Domains of an H-NS molecule, from the reference. The oligomerization domain binds to a fellow H-NS while the DNA binding domain attaches to DNA.

Whatever the binding mechanism is, it is unrelated to the actual sequence of the DNA, as H-NS can bind to many different regions of the gene, regardless of sequence. Until recently the H-NS molecules were thought to have no post-translational modifications (unlike histones which are often decorated in molecular markers to indicate which kind of gene they are on) but some have recently been found to be marked with poly-3-hydroxybutyrate, a small lipid molecule. The reason for this is unclear, but it does raise some exciting implications for H-NS control of gene transcription.

When it comes to controling DNA expression, it's clear that in both eukaryotes and prokaryotes, the scaffold proteins that hold the DNA coiled up act as more than just a scaffold. Instead they are involved in a substantial amount of the control of DNA expression, often working closely with other control proteins to ensure the correct genes are turned into proteins.


Schröder O, & Wagner R (2002). The bacterial regulatory protein H-NS--a versatile modulator of nucleic acid structures. Biological chemistry, 383 (6), 945-60 PMID: 12222684

Follow me on Twitter!

No comments: