Formation of hydrogen sulfide from cysteine in Saccharomyces cerevisiae BY4742: genome wide screen reveals a central role of the vacuole

Discoveries on the toxic effects of cysteine accumulation and, particularly, recent findings on the many physiological roles of one of the products of cysteine catabolism, hydrogen sulfide (H₂S), are highlighting the importance of this amino acid and sulfur metabolism in a range of cellular activities. It is also highlighting how little we know about this critical part of cellular metabolism. In the work described here, a genome-wide screen using a deletion collection of Saccharomyces cerevisiae revealed a surprising set of genes associated with this process. In addition, the yeast vacuole, not previously associated with cysteine catabolism, emerged as an important compartment for cysteine degradation. Most prominent among the vacuole-related mutants were those involved in vacuole acidification; we identified each of the eight subunits of a vacuole acidification sub-complex (V₁ of the yeast V-ATPase) as essential for cysteine degradation. Other functions identified included translation, RNA processing, folate-derived one-carbon metabolism, and mitochondrial iron-sulfur homeostasis. This work identified for the first time cellular factors affecting the fundamental process of cysteine catabolism. Results obtained significantly contribute to the understanding of this process and may provide insight into the underlying cause of cysteine accumulation and H₂S generation in eukaryotes.

Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends

From 1979 to 2001, temperatures observed globally by the mid-tropospheric channel of the satellite-borne Microwave Sounding Unit (MSU channel 2), as well as the inferred temperatures in the lower troposphere, show only small warming trends of less than 0.1 K per decade (refs 1-3). Surface temperatures based on in situ observations however, exhibit a larger warming of approximately 0.17 K per decade (refs 4, 5), and global climate models forced by combined anthropogenic and natural factors project an increase in tropospheric temperatures that is somewhat larger than the surface temperature increase. Here we show that trends in MSU channel 2 temperatures are weak because the instrument partly records stratospheric temperatures whose large cooling trend offsets the contributions of tropospheric warming. We quantify the stratospheric contribution to MSU channel 2 temperatures using MSU channel 4, which records only stratospheric temperatures. The resulting trend of reconstructed tropospheric temperatures from satellite data is physically consistent with the observed surface temperature trend. For the tropics, the tropospheric warming is approximately 1.6 times the surface warming, as expected for a moist adiabatic lapse rate.

Interpreting differential temperature trends at the surface and in the lower troposphere

Estimated global-scale temperature trends at Earth's surface (as recorded by thermometers) and in the lower troposphere (as monitored by satellites) diverge by up to 0.14 degrees C per decade over the period 1979 to 1998. Accounting for differences in the spatial coverage of satellite and surface measurements reduces this differential, but still leaves a statistically significant residual of roughly 0.1 degrees C per decade. Natural internal climate variability alone, as simulated in three state-of-the-art coupled atmosphere-ocean models, cannot completely explain this residual trend difference. A model forced by a combination of anthropogenic factors and volcanic aerosols yields surface-troposphere temperature trend differences closest to those observed.

Multidecadal changes in the vertical temperature structure of the tropical troposphere

Trends in global lower tropospheric temperature derived from satellite observations since 1979 show less warming than trends based on surface meteorological observations. Independent radiosonde observations of surface and tropospheric temperatures confirm that, since 1979, there has been greater warming at the surface than aloft in the tropics. Associated lapse-rate changes show a decrease in the static stability of the atmosphere, which exceeds unforced static stability variations in climate simulations with state-of-the-art coupled ocean-atmosphere models. The differential temperature trends and lapse-rate changes seen during the satellite era are not sustained back to 1960.

Trends in the vertical distribution of ozone

Analyses of satellite, ground-based, and balloon measurements allow updated estimates of trends in the vertical profile of ozone since 1979. The results show overall consistency among several independent measurement systems, particularly for northern hemisphere midlatitudes where most balloon and ground-based measurements are made. Combined trend estimates over these latitudes for the period 1979-96 show statistically significant negative trends at all altitudes between 10 and 45 km, with two local extremes: -7.4 +/- 2. 0% per decade at 40 km and -7.3 +/- 4.6% per decade at 15 km altitude. There is a strong seasonal variation in trends over northern midlatitudes in the altitude range of 10 to 18 km, with the largest ozone loss during winter and spring. The profile trends are in quantitative agreement with independently measured trends in column ozone, the amount of ozone in a column above the surface. The vertical profiles of ozone trends provide a fingerprint for the mechanisms of ozone depletion over the last two decades.