Control of Hexamerization, Assembly, and Excluded Strand Specificity for the Sulfolobus solfataricus MCM Helicase.

A growing body of evidence supports a steric exclusion and wrapping model for DNA unwinding in which hexameric helicases interact with the excluded single-stranded DNA (ssDNA) in addition to the encircled strand. Interactions with the excluded ssDNA have been shown to be mediated primarily by electrostatic interactions, but base stacking with surface-exposed tyrosine residues is an alternative hypothesis. Here, we mutated several external tyrosine and positively charged residues from full-length Sulfolobus solfataricus MCM along the proposed path of excluded strand binding and assessed their impact on DNA unwinding. Four of the five tyrosine residues had significant decreases in their level of unwinding, and one, Y519A, located within the α/ß-α linker region of the C-terminal domain, had the most severe perturbation attributed to the disruption of hexamerization. The Y519 mutant exhibits an enhanced and stabilized secondary structure that is modulated by temperature, binding DNA with a higher apparent affinity and suggesting a pathway for hexameric assembly. Hydrogen/deuterium exchange coupled to mass spectrometry was used to map deuterium uptake differences between wild-type and Y519A apo structures highlighting global differences in solvent accessible areas consistent with altered quaternary structure. Two of the five electrostatic mutants had significantly reduced levels of DNA unwinding and combined with previous mutations better define the exterior binding path. The importance of the electrostatic excluded strand interaction was confirmed by use of morpholino DNA substrates that showed analogous reduced unwinding rates. These results better define the hexameric assembly and influence of the excluded strand interactions in controlling DNA unwinding by the archaeal MCM complex.

Effect of nature exposure on perceived and physiologic stress: A systematic review.

OBJECTIVE: The purpose of this systematic review is to evaluate the effect nature exposure has on stress as measured by physiologic markers and self-report. DESIGN: Researchers searched PubMed and JSTOR. Randomized control trials and cross-sectional studies were included if they met the following criteria: 1) included a clinical cohort and controls, and the intervention was nature exposure, either real or simulated; (2) utilized measurements of sympathetic activity or perceived stress; (3) study population consisted of greater than thirty male and female volunteers. Twelve studies were included for data extraction and review. SETTING: Researchers conducted this review at University of South Florida Morsani College of Medicine. MAIN OUTCOME MEASURES: Researchers measured perceived stress through the Perceived Stress Scale (PSS) and Depression, Anxiety, and Stress Scale (DASS). Physiologic stress was measured by salivary cortisol, blood pressure (BP), subgenual prefrontal cortex (sgPFC) activation on functional magnetic resonance imaging (fMRI), skin conductance level (SCL), heart rate variability (HRV), muscle tension, heart period, pulse transit time, amygdala and pregenual anterior cingulate cortex (pACC) activation on fMRI, respiratory sinus arrhythmia (RSA), and pre-ejection period (PEP). RESULTS: This review found an inverse relationship between nature exposure and various physiologic markers of stress in all 7 studies measuring physiologic stress. Perceived stress was also affected by higher exposure to nature as indicated by lower self-reports on the PSS and DASS in 5 out of 6 studies measuring perceived stress. CONCLUSIONS: Nature exposure has been widely shown to have a positive effect in reducing stress, both perceived and physiologic.