Imaging and energetics of single SSB-ssDNA molecules reveal intramolecular condensation and insight into RecOR function.

Escherichia coli single-stranded DNA (ssDNA) binding protein (SSB) is the defining bacterial member of ssDNA binding proteins essential for DNA maintenance. SSB binds ssDNA with a variable footprint of ∼30-70 nucleotides, reflecting partial or full wrapping of ssDNA around a tetramer of SSB. We directly imaged single molecules of SSB-coated ssDNA using total internal reflection fluorescence (TIRF) microscopy and observed intramolecular condensation of nucleoprotein complexes exceeding expectations based on simple wrapping transitions. We further examined this unexpected property by single-molecule force spectroscopy using magnetic tweezers. In conditions favoring complete wrapping, SSB engages in long-range reversible intramolecular interactions resulting in condensation of the SSB-ssDNA complex. RecO and RecOR, which interact with SSB, further condensed the complex. Our data support the idea that RecOR--and possibly other SSB-interacting proteins-function(s) in part to alter long-range, macroscopic interactions between or throughout nucleoprotein complexes by microscopically altering wrapping and bridging distant sites

DNA unwinding heterogeneity by RecBCD results from static molecules able to equilibrate.

Single-molecule studies can overcome the complications of asynchrony and ensemble-averaging in bulk-phase measurements, provide mechanistic insights into molecular activities, and reveal interesting variations between individual molecules. The application of these techniques to the RecBCD helicase of Escherichia coli has resolved some long-standing discrepancies, and has provided otherwise unattainable mechanistic insights into its enzymatic behaviour. Enigmatically, the DNA unwinding rates of individual enzyme molecules are seen to vary considerably, but the origin of this heterogeneity remains unknown. Here we investigate the physical basis for this behaviour. Although any individual RecBCD molecule unwound DNA at a constant rate for an average of approximately 30,000 steps, we discover that transiently halting a single enzyme-DNA complex by depleting Mg(2+)-ATP could change the subsequent rates of DNA unwinding by that enzyme after reintroduction to ligand. The proportion of molecules that changed rate increased exponentially with the duration of the interruption, with a half-life of approximately 1 second, suggesting that a conformational change occurred during the time that the molecule was arrested. The velocity after pausing an individual molecule was any velocity found in the starting distribution of the ensemble. We suggest that substrate binding stabilizes the enzyme in one of many equilibrium conformational sub-states that determine the rate-limiting translocation behaviour of each RecBCD molecule. Each stabilized sub-state can persist for the duration (approximately 1 minute) of processive unwinding of a DNA molecule, comprising tens of thousands of catalytic steps, each of which is much faster than the time needed for the conformational change required to alter kinetic behaviour. This ligand-dependent stabilization of rate-defining conformational sub-states results in seemingly static molecule-to-molecule variation in RecBCD helicase activity, but in fact reflects one microstate from the equilibrium ensemble that a single molecule manifests during an individual processive translocation event

Envelope Expansion with Core Collapse. III. Similarity Isothermal Shocks in a Magnetofluid

We explore MHD solutions for envelope expansions with core collapse (EECC) with isothermal MHD shocks in a quasi-spherical symmetry and outline potential astrophysical applications of such magnetized shock flows. MHD shock solutions are classified into three classes according to the downstream characteristics near the core. Class I solutions are those characterized by free-fall collapses towards the core downstream of an MHD shock, while Class II solutions are those characterized by Larson-Penston (LP) type near the core downstream of an MHD shock. Class III solutions are novel, sharing both features of Class I and II solutions with the presence of a sufficiently strong magnetic field as a prerequisite. Various MHD processes may occur within the regime of these isothermal MHD shock similarity solutions, such as sub-magnetosonic oscillations, free-fall core collapses, radial contractions and expansions. We can also construct families of twin MHD shock solutions as well as an `isothermal MHD shock' separating two magnetofluid regions of two different yet constant temperatures. The versatile behaviours of such MHD shock solutions may be utilized to model a wide range of astrophysical problems, including star formation in magnetized molecular clouds, MHD link between the asymptotic giant branch phase to the proto-planetary nebula phase with a hot central magnetized white dwarf, relativistic MHD pulsar winds in supernova remnants, radio afterglows of soft gamma-ray repeaters and so forth.Comment: 21 pages, 33 figures, accepted by MNRA

Can green LED light do a magical thing to plants?

Red and blue light are the most important in driving photosynthesis to produce adequate yield. It is also believed that green light may contribute to the adaptation to growth. However, the effects of the green light which may trigger specific and necessary responses in plant growth have been underestimated in the past. In this study, lettuce (Lactuca sativa L.) was exposed to different continuous light (CL) conditions for 48 h by combination of red and blue light emitting diodes (LEDs) supplied with/without green LEDs in an environmental controlled growth chamber. Green light supplementation enhanced photosynthetic capacity by increasing net photosynthetic rate (P n), maximal photochemical efficiency (Fv/Fm), electron transport for carbon fixation (JSPII) and chlorophyll content, which led to increases of plant fresh and dry weight under CL treatment. Green light decreased malondialdehyde and H2O2 accumulation by increasing superoxide dismutase (SOD) enzyme, catalase (CAT) enzyme and ascorbate peroxidase (APX) activities after 24 h CL. Supplementary green light was also shown to lead to a significant increase in the expression of the photosynthetic genes Lhcb and psbA from 6 to 12 h and retained higher level compared with other light conditions between 12 and 24 h. The results indicate that the effects of green light on the lettuce plant growth via promoting psbA and Lhcb expression to maintain higher photosynthetic capacity and green light could alleviate the negative effects caused by CL

Continuous light-emitting Diode (LED) lighting for improving food quality

Lighting-emitting diodes (LEDs) have shown great potential for plant growth and development, with higher luminous efficiency and positive impact compared with other artificial lighting. The combined effects of red/blue or/and green, and white LED light on plant growth and physiology, including chlorophyll fluorescence, nitrate content and phytochemical concentration before harvest, were investigated. The results showed that continuous light (CL) exposure at pre-harvest can effectively reduce nitrate accumulation and increase phytochemical concentrations in lettuce plants, and the former is dependent on the spectral composition and continuous light duration. Lettuce plants grown under continuous combined red and blue (with or without green) LED light with a photosynthetic photon flux density (PPFD) at 200 μ mol m-2-s-1 exhibited a remarkable decrease of nitrate contents at 24 h compared to other light treatments. In addition, red and blue light (R:B=4:1) was more effective in facilitating lettuce growth than white LED light at the same PPFD. Moreover, continuous LED light for 24 h significantly enhanced the free-radical scavenging activity and increased phenolic compound concentrations. In this study, we suggest that a period of continuous LED (R/B) with green (G) light exposure is needed in order to decrease nitrate concentrations and enhance lettuce quality. 24 h appears to be the best, but this period should not exceed 48 h. It appears that continuous light could enhance the activity of nitrate reductase leading to a low level of nitrate content in the leaf. However, the reduction of nitrate is considered to be associated with the circadian clock and the light-signaling pathway as well