A real-time single molecule approach to understand how DNA repair proteins locate and remove damage

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From microbes to man DNA repair is crucial to the continuance of life. Each cell in the human body accumulates over 10000 sites of DNA damage every day, therefore efficient and rapid repair is essential. Defects in DNA repair result in cell death or continual proliferation, leading to premature ageing or tumour formation respectively. Repair is mediated by proteins; each one performs a small task in a sequence that eventually leads to lesion repair. To date we do not fully understand the physical basis of how these proteins find damage or come together as functional units. In this project we aim to follow the process of nucleotide excision repair (NER) in a model bacterial system. This simpler system involves the interplay of just three dedicated enzymes instead of up to thirty in humans. We will use recent advances in imaging such as fast sensitive cameras, bright fluorescent tags and powerful computers to directly visualise how and when these protein machines operate; this is discussed in more detail below. Our research offers important insights into how proteins find their targets on DNA, form complexes and permits direct visualisation of the mechanistic sequence underlying a protein cascade. It is anticipated that this research will benefit other scientists by introducing new techniques that could be used to investigate a number of other processes and may also impact the design of new anti-bacterial drugs. To study DNA repair we visualise the process one molecule at a time. Normally, systems are studied as 'ensembles' consisting of thousands of billions of protein molecules. By visualising single molecules we are able to extract information much more accurately about both the order and timing of the process being studied. To make it possible to see a single molecule we attach fluorescent beacons called a quantum dots to our proteins. These tagged proteins can then be followed using a state-of-the-art microscope based imaging technique. However to follow the proteins one more important aspect needs to be considered. When DNA is visualised it is not a long stretched out fibre, instead DNA is bundled, making it impossible to follow the behaviour of a single tagged protein. To overcome this we have developed a unique approach: we suspend the DNA between large beads attached to a microscope slide to create 'DNA tightropes'. These tightropes allow us to introduce tagged proteins and watch how they behave on DNA. Since the repair system uses multiple protein machines to carry out its work, we have tagged the proteins with different colours to distinguish them. DNA repair proteins face the enormous 'needle in a haystack' challenge of finding one damage site amongst a vast excess (millions to one) of undamaged DNA. Using our tightrope technology we will watch how they do this, and at the same time make precise measurements to provide us with a physical understanding of this process. Do the proteins slide along the DNA? Detach and reattach elsewhere? Or both? We will also be able to address long held questions in the field such as how many proteins form a complex? And what role ATP, the cellular energy currency, plays? We will also damage the strung up DNA tightropes and attach a quantum dot beacon to the damage site thus providing us with its location. Then we will introduce all three proteins together and, in real time, we will directly observe how they work together to repair the DNA. In this proposal we present a large amount of data to demonstrate the success of the above outlined approach, which uses technology that is at the leading edge of the field and is unique to our laboratory. The system we are developing here will offer a new insight into DNA repair and also provide enabling technology to offer a new way of understanding how many other protein systems interact with DNA.