Single-molecule FRET-Rosetta reveals RNA structural rearrangements during human telomerase catalysis.

Maintenance of telomeres by telomerase permits continuous proliferation of rapidly dividing cells, including the majority of human cancers. Despite its direct biomedical significance, the architecture of the human telomerase complex remains unknown. Generating homogeneous telomerase samples has presented a significant barrier to developing improved structural models. Here we pair single-molecule Förster resonance energy transfer (smFRET) measurements with Rosetta modeling to map the conformations of the essential telomerase RNA core domain within the active ribonucleoprotein. FRET-guided modeling places the essential pseudoknot fold distal to the active site on a protein surface comprising the C-terminal element, a domain that shares structural homology with canonical polymerase thumb domains. An independently solved medium-resolution structure of Tetrahymena telomerase provides a blind test of our modeling methodology and sheds light on the structural homology of this domain across diverse organisms. Our smFRET-Rosetta models reveal nanometer-scale rearrangements within the RNA core domain during catalysis. Taken together, our FRET data and pseudoatomic molecular models permit us to propose a possible mechanism for how RNA core domain rearrangement is coupled to template hybrid elongation.

Positive impact of a breast-health triaging system on breast-care access and physician satisfaction.

OBJECTIVE: Minority patients are at risk for delayed breast cancer treatment. Using nonsurgical breast specialists could improve access but requires appropriate referral to ensure prompt cancer care. Our objective was to evaluate a referral triage system in a combined medical/surgical breast health program (BHP). METHODS: A triage system based on imaging findings, examination, and patient age was instituted. An advanced practice nurse managed referrals and a prospective database. Referring providers were surveyed after 2 years. RESULTS: From 2003 to 2006, 4,840 referrals were made to surgeons (57%) and nonsurgeons (43%). Breast cancers were found in 8.5% of patients. Referral error occurred in 4 cancer patients (.1%). BHP-referred patients had significantly shorter times to surgical appointment (10 days) than non-BHP referrals (45 days). A referring provider survey indicated 96% satisfaction. CONCLUSIONS: A breast-care triage system expedited cancer care resulting in physician satisfaction and increased referrals.

Influence of breast cancer margin assessment method on the rates of positive margins and residual carcinoma.

BACKGROUND: We hypothesized that the method of breast cancer margin assessment may be associated with different rates of positive margins and residual carcinoma. METHODS: A total of 178 breast cancer specimens were divided into 2 groups (A and B) based on the margin assessment method used. Rates of positive margins, re-excision, and residual carcinoma at re-excision were compared and analyzed statistically. RESULTS: At least 1 margin was positive in 64.7% in group A and in 65.2% in group B. At directed re-excision 54% in group A and 51% in group B had residual carcinoma. The lateral margin was positive in 44% in group A compared with 26% in group B (P = .06). The posterior margin was positive in 19% in group A and in 51% in group B (P = .001). CONCLUSIONS: Two different breast cancer specimen margin assessment methods had comparable rates of positive margins and residual carcinoma at re-excision. Different patterns of specific margin positivity suggest that the method of margin assessment may alter results.

Chirality sensing by Escherichia coli topoisomerase IV and the mechanism of type II topoisomerases.

Escherichia coli topoisomerase (Topo) IV is an essential type II Topo that removes DNA entanglements created during DNA replication. Topo IV relaxes (+) supercoils much faster than (-) supercoils, promoting replication while sparing the essential (-) supercoils. Here, we investigate the mechanism underlying this chiral preference. Using DNA binding assays and a single-molecule DNA braiding system, we show that Topo IV recognizes the chiral crossings imposed by the left-handed superhelix of a (+) supercoiled DNA, rather than global topology, twist deformation, or local writhe. Monte Carlo simulations of braid, supercoil, and catenane configurations demonstrate how a preference for a single-crossing geometry during strand passage can allow Topo IV to perform its physiological functions. Single-enzyme braid relaxation experiments also provide a direct measure of the processivity of the enzyme and offer insight into its mechanochemical cycle.