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In cocrystal structures of human topoisomerase I and DNA, the enzyme is tightly clamped around the DNA helix. After cleavage and covalent attachment of the enzyme to the 3 end at the nick, DNA relaxation requires rotation of the DNA helix downstream of the cleavage site. Models based on the cocrystal structure reveal that there is insufficient space in the protein for such DNA rotation without some deformation of the cap and linker regions of the enzyme. Alternatively, it is conceivable that the protein clamp opens to facilitate the rotation process. To distinguish between these two possibilities, we engineered two cysteines into the opposing loops of the ''lips'' region of the enzyme, which allowed us to lock the protein via a disulfide crosslink in the closed conformation around the DNA. Importantly, the rate of DNA relaxation when the enzyme was locked on the DNA was comparable to that observed in the absence of the disulfide crosslink. These results indicate that DNA relaxation likely proceeds without extensive opening of the enzyme clamp. D NA topoisomerases are ubiquitous and essential enzymes that solve the topological problems accompanying key nuclear processes such as DNA replication, transcription, repair, and chromatin assembly by introducing temporary single-or double-strand breaks in the DNA (1-4). In addition, these enzymes fine-tune the steady-state level of DNA supercoiling to facilitate protein interactions with DNA and to prevent excessive supercoiling that is deleterious. There are two fundamental types of topoisomerases, which differ in both mechanism and cellular function (1). Type II enzymes are dimeric, promote the passage of one region of duplex DNA through a double-stranded break in the same or a different molecule, and are primarily dedicated to such processes as DNA supercoiling and chromosome segregation. Type I enzymes are monomeric and transiently break one strand of the duplex DNA, allowing for adjustments in helical winding. These enzymes are primarily responsible for removing torsional stress generated by processes that leave the DNA overwound or underwound. The type I topoisomerases have been further divided into two subfamilies based on sequence comparisons and reaction mechanisms (2). The type IA subfamily is characterized by covalent attachment of the enzyme to the 5Ј end of the broken strand in the nicked intermediate, whereas all members of the type IB subfamily attach to the 3Ј end of the broken strand. The prototype of the type IB subfamily, human topoisomerase I, catalyzes changes in the superhelical state of duplex DNA by transiently breaking one strand of the DNA to allow rotation of one region of the duplex relative to another region (5). Strand cleavage is achieved by the nucleophilic attack of the active site tyrosine on a DNA phosphodiester bond. The resulting formation of a phosphodiester bond between the tyrosine and the 3Ј end of the cleaved strand enables the enzyme to reseal the DNA by simple reversal of the cleavage reaction (1).Human topoisomerase I is a 765...

The double burden of suboptimal nutrient intake and obesity exists when available foods lack essential nutrients to promote health and provide high amounts of energy. This study evaluated the nutrition content of 41 meals served to the homeless at 3 urban soup kitchens. The mean nutrient content of all meals and of meals from each of the kitchens was compared to two-thirds of the estimated average requirement (EAR

Food recovery programs are an increasingly popular method to decrease the amount of food waste in the United States while increasing healthy food access for food insecure individuals. One such program is the Heartside Gleaning Initiative that began in 2014 in Grand Rapids, Michigan. The program partners with low-income volunteers to collect fresh produce from farmers' markets for redistribution. From June to October 2014, 17,000 pounds of fresh produce was provided to low-income individuals and food pantries. A survey of participants indicated high usage of the produce and high satisfaction with the gleaning program. This program confirms that gleaning is a viable method of reducing barriers to accessing produce for food pantries and food insecure individuals.