Escherichia coli Exonuclease I (ExoI) digests single-stranded DNA (ssDNA) in the 3′-5′ direction in a highly processive manner. The crystal structure of ExoI, determined previously in the absence of DNA, revealed a C-shaped molecule with three domains that form a central positively charged groove. The active site is at the bottom of the groove, while an extended loop, proposed to encircle the DNA, crosses over the groove. Here, we present crystal structures of ExoI in complex with four different ssDNA substrates. The structures all have the ssDNA bound in essentially the predicted manner, with the 3′-end in the active site and the downstream end under the crossover loop. The central nucleotides of the DNA form a prominent bulge that contacts the SH3-like domain, while the nucleotides at the downstream end of the DNA form extensive interactions with an ‘anchor’ site. Seven of the complexes are similar to one another, but one has the ssDNA bound in a distinct conformation. The highest-resolution structure, determined at 1.95 Å, reveals an Mg2+ ion bound to the scissile phosphate in a position corresponding to MgB in related two-metal nucleases. The structures provide new insights into the mechanism of processive digestion that will be discussed.

Our work does not support an independent influence of PTH on cognitive decline in this population-based cohort study.

Background and aims Frailty and cardiovascular disease share many risk factors. We evaluated whether frailty is independently associated with subclinical coronary atherosclerosis and whether any relationships differ by HIV-serostatus. Methods We studied 976 [62% HIV-infected] male participants of the Multicenter AIDS Cohort Study who underwent assessment of frailty and non-contrast cardiac CT scanning; of these, 747 men also underwent coronary CT angiography (CCTA). Frailty was defined as having ≥3 of 5 of the following: weakness, slowness, weight loss, exhaustion, and low physical activity. Coronary artery calcium (CAC) was assessed by non-contrast CT, and total plaque score (TPS), mixed plaque score (MPS), and non-calcified plaque score (NCPS) by CCTA. Multivariable-adjusted regression was used to assess the cross-sectional associations between frailty and subclinical coronary atherosclerosis. Results Mean (SD) age of participants was 54 (7) years; 31% were black. Frailty existed in 7.5% and 14.3% of HIV-uninfected and HIV-infected men, respectively. After adjustment for demographics, frailty was significantly associated with prevalence of any CAC (CAC>0), any plaque (TPS>0), and mixed plaque (MPS>0) in HIV-uninfected but not in HIV-infected men (p-interaction_HIV<0.05 for all). Among HIV-uninfected men, after adjustment for cardiovascular risk factors, frailty was significantly associated only with CAC>0 [Prevalence Ratio 1.27 (95%CI 1.02, 1.59)] and TPS>0 [1.19 (1.06, 1.35)]. No association was found for NCPS. Conclusions Frailty was independently associated with subclinical coronary atherosclerosis among HIV-uninfected men, but not among HIV-infected men. Further work is needed to ascertain mechanisms underlying these differences and whether interventions that improve frailty (i.e. strength training) can improve cardiovascular outcomes.

Background Elevated parathyroid hormone (PTH) levels have been associated with cardiovascular disease risk factors and events. We hypothesized that elevated PTH would also be associated with subclinical cerebrovascular disease. We examined the relationship of elevated PTH with white matter hyperintensities (WMH) and subclinical infarcts measured on brain MRI. Methods PTH was measured at baseline (1993–1994) among participants free of prior clinical stroke who underwent a brain MRI at baseline (n=1703) and a second brain MRI 10 years later (n=948). PTH levels ≥65 pg/ml were considered elevated (n=204). Participants who did not return for a follow-up MRI had, at baseline, higher PTH and a greater prevalence of cardiovascular risk factors (p<0.05 for all); therefore multiple imputation was used. The cross-sectional and prospective associations of PTH levels with WMH and MRI-defined infarcts (and their progression) were investigated using multivariable regression models. Results At baseline, participants were mean age of 62 years, 60% female and 49% black. Cross-sectionally, after adjusting for demographic and lifestyle factors, elevated PTH was associated with higher WMH score [β=0.19, 95%CI 0.04–0.35] and increased odds of prevalent infarcts [OR 1.56,1.02–2.36]. Results were attenuated after adjustment for potential mediators of this association (i.e. hypertension). No prospective associations were found between PTH and incident infarcts or change in estimated WMH volume, although estimates were imprecise. Conclusions Although associated cross-sectionally, we did not confirm any association of elevated PTH with progression of cerebrovascular changes on brain MRIs obtained 10 years apart. The relationship of PTH with subclinical brain disease warrants further study.