Capturing of the monoterpene olefin limonene produced in Saccharomyces cerevisiae

Monoterpene olefins such as limonene are plant compounds with applications as flavouring and fragrance agents, as solvents and potentially also in polymer and fuel chemistry. We engineered baker's yeast Saccharomyces cerevisiae to express a (?)‐limonene synthase from Perilla frutescens and a (+)‐limonene synthase from Citrus limon. Both proteins were expressed either with their native plastid targeting signal or in a truncated form in which the plastidial sorting signal was removed. The yeast host strain for expression was AE9 K197G, which expresses a mutant Erg20 enzyme. This enzyme catalyses the formation of geranyl diphosphate, which is the precursor for monoterpenes. Several methods were tested to capture limonene produced by the yeast. Extraction from the culture medium by pentane, or by the addition of CaCl₂ followed by solid‐phase micro‐extraction, did not lead to detectable limonene, indicating that limonene is rapidly lost from the culture medium. Volatile terpenes such as limonene may also be trapped in a dodecane phase added to the medium during fermentation. This method resulted in recovery of 0.028 mg/l (+)‐limonene and 0.060 mg/l (?)‐limonene in strains using the truncated Citrus and Perilla synthases, respectively. Trapping the headspace during culture of the limonene synthase‐expressing strains resulted in higher titres, at 0.12 mg/l (+)‐limonene and 0.49 mg/l (?)‐limonene. These results show that the volatile properties of the olefins produced require specific methods for efficient recovery of these molecules from biotechnological production systems. Gene Bank Nos were: KM015220 (Perilla limonene synthase; this study); AF317695 (Perilla limonene synthase; Yuba et al., 1996 ); AF514287.1 (Citrus limonene synthase; Lucker et al., 2002 ). Copyright © 2014 John Wiley & Sons, Ltd.     

Characteristics of Rainfall, Snowmelt and Runoff in the Headwater Region of the Main River Watershed in Germany

In the headwater region of the Main River watershed in Germany, floods resulted from rain-on-snow events often occur in winter. Data of long-term observations at 16 gauging stations and 11 climate stations are available. Using these data, the objective of this paper is to study the characteristics of precipitation including snow depth and snow water equivalent (SWE). The importance of rainfall on the snow melt process has been assessed. Statistical analysis and trend analysis of extreme precipitation, snow depth, SWE and river discharges have been carried out. Through introducing equivalent precipitation depth from snowmelt with rainfall as a new variable, the simultaneous occurrence of snowmelt and rain-on-snow has been assessed; the characteristics of runoff including peak discharge caused by rain-on-snow have been investigated. The major climatic feature was found to be a precipitation, which as maximum in summer; and the major hydrological feature was a discharge, which was maximum in winter. By using two different models, the Unit Hydrographs at some gauging stations have been determined and compared.     

Modeling the axial extension of a transmission line source within iterative reconstruction via multiple transmission sources

Reconstruction algorithms for transmission tomography have generally assumed that the photons reaching a particular detector bin at a particular angle originate from a single point source. In this paper, we highlight several cases of extended transmission sources, in which it may be useful to approach the estimation of attenuation coefficients as a problem involving multiple transmission point sources. Examined in detail is the case of a fixed transmission line source with a fan-beam collimator. This geometry can result in attenuation images that have significant axial blur. Herein it is also shown, empirically, that extended transmission sources can result in biased estimates of the average attenuation, and an explanation is proposed. The finite axial resolution of the transmission line source configuration is modeled within iterative reconstruction using an expectation-maximization algorithm that was previously derived for estimating attenuation coefficients from single photon emission computed tomography (SPECT) emission data. The same algorithm is applicable to both problems because both can be thought of as involving multiple transmission sources. It is shown that modeling axial blur within reconstruction removes the bias in the average estimated attenuation and substantially improves the axial resolution of attenuation images.