The cys2-1 mutation of Saccharomyces cerevisiae was originally thought to confer cysteine dependence through a serine O-acetyltransferase deficiency. In this study, we show that cys2-1 strains lack not only serine O-acetyltransferase but also'cystathionine f-synthase. However, a -prototrophic strain was found to be serine O-acetyltransferase deficient because of a mutation allelic to cys2-1. Moreover, revertants obtained from cys2-1 strains had serine O-acetyltransferase but not cystathionine P-synthase, whereas transformants obtained by treating a cys2-1 strain with an S. cerevisiae genomic library had cystathionine ,-synthase but not serine O-acetyltransferase. From these observations, we conclude that cys2-1 (serine O-acetyltransferase deficiency) accompanies a very closely linked mutation that causes cystathionine I-synthase deficiency and that these mutations together confer cysteine dependence. This newly identified mutation is named cys4-1. These results not only support our previous hypothesis that S. cerevisiae has two functional cysteine biosynthetic pathways but also reveal an interesting gene arrangement of the cysteine biosynthetic system. Current understanding of cysteine and methionine biosynthesis in Saccharomyces cerevisiae is illustrated in Fig. 1 (2,15,25). On the other hand, Halos (cited in reference 11) obtained mutants that grew only on media supplemented with cysteine, found that they were serine O-4cetyltransferase (EC 2.3

Hg2 -resistant mutants were isolated from Saccharomyces cerevisiae. Although they were very much like the parental strains in terms of colony-forming ability, they grew faster than the parental strains in the presence of sublethal doses of Hg2+. The Hg2+-resistant mutations were dominant. They were centromere linked and were divided into two groups by means of recombination; one of the mutations, designated HGRI-1, was mapped on chromosome IV because of its linkage to the TRPI locus. The Hg2+-resistant mutants took up Hg2+ as much as, or slightly more than, the parental strains did. The mutants and parental strains retained only about 5 and 15%, respectively, of the cell-associated Hg2+ after removal of the cell wall; therefore, the mutants had less spheroplast-associated Hg2+ than did the parental strains. These results indicate that the cell wall plays an important role in protection against Hg2+ by acting as an adsorption filter and that the mutations described confer Hg2+ resistance by increasing the Hg2+-binding capacity of the cell wall.For a better understanding of the toxicity of mercury and mechanisms of biological defense against mercury, studies at the cellular and subcellular levels are indispensable. Saccharomyces cerevisiae is suitable for such studies because it is easily manipulated genetically and readily analyzed biochemically. Singh and Sherman (17, 18) isolated mutants resistant to methylmercury and showed that the methylmercury-resistant mutations arose in only the ME72 and METI5 loci. On the other hand, mutants resistant to inorganic mercury were obtained by training; i.e., a wild-type strain was cultured in medium containing a sublethal concentration of mercury, and then the mercury concentration was gradually increased as the cells grew (5, 22). However, the resultant mutants were so unstable that they quickly lost resistance when they were cultured without mercury.We have been studying the toxicity of Hg2+ and the biological defense against Hg2+ by using S. cerevisiae. It has been shown that the Hg2+ sensitivity of a certain strain is caused by the joint action of two mutations (10). One of them blocks tyrosine biosynthesis, while the other blocks tyrosine uptake by enhancing catabolite repression of the tyrosine uptake system (11, 13). Hg2+ promotes depletion of cellular tyrosine by inhibiting the tyrosine uptake system (13). To search for other biological effects of Hg2 + we attempted to obtain stable Hg2+-resistant mutants. In this report, we describe the isolation and partial characterization of the mutants. From these results, we discuss a role of the cell wall in protection against Hg2+ in S. cerevisiae. MATERIALS AND METHODSGrowth media, strains, and isolation of mutants. YPD medium contained 1% yeast extract, 2% peptone, and 2% glucose (15). For solid media, 2% agar was added. Mutants were isolated from strain EH1-4A (MATot leu2-1 met8-1 gal2 msml-2) and EH1-4B (MATa leu2-1 met8-1 his5-2 MSMJ-1). was placed at the center of each plate, and 2.1 mg of HgCl2 in solution was applied ...

The Laser Doppler Velocimetry (LDV) is employed to investigate energy dissipation during a spin-down process inside a rotating drum. The tracer/light sheet method is applied to observe flow patterns in the entire flow field from which the instantaneous, two-dimensional velocity distribution and the formation and subsequent time wise variation of the Ekman boundary layer are determined. Results are synthesized to find the relationship between the Ekman boundary layer and the redistribution of secondary-flow induced angular momentum. The fluid viscosity, drum size and speed of rotation are varied to determine their effects on both the Ekman boundary layer and energy dissipation during spin-down process. The role of Ekman boundary layer in the reduction of rotating fluid motion is determined. Results from the study may be used to develop a method to achieve uniform mixing in an enclosed vessel.

To reduce the molecular diffusion mixing time in a microreactor, the efficacy of a layered flow pattern generated by an alternate pumping system has been reported. In this paper, we propose an asterisk-shaped channel, which has two inlets and two diverging mixing channels, for the enhancement of mixing using the layered structure flow pattern. Using scaled-up models, the characteristics of the flow pattern in the asteriskshaped channel was investigated in association with the pumping condition and the channel geometry. Because the gap between the inlets is small in the asterisk-shaped channel, a layered structure is generated under easy pumping condition, which could be practicable. The layer thickness in the diverging channel becomes smaller downstream, so the diverging channels reduce the molecular diffusion time that is proportional to the square of the mixing length, i.e. the layer thickness in this case. For instance, in the case that the width and length of the mixing channel are 5 times and 20 times of the inlet width, the mixing time in the asterisk channel with alternate pumping could be reduced to two orders of magnitude smaller than that in an ordinary microreactor.

This paper elucidates the performance degradation and flow instability of an axial fan caused by the presence of disk-shaped obstacles upstream of the fan, such as wall surfaces. The increase in pressure loss and the decrease in shaft power coefficient due to inlet swirl flow, and the increase in pressure loss due to the outlet swirl flow, cause performance degradation. When the obstacle is closer to the fan, the strong swirl flow causes a negative pressure region between the fan and the obstacle, reversing the flow direction. This phenomenon is caused by the diffuser effect of the outward flow and the increase in pressure by acting as a multiblade centrifugal fan. At a low flow rate, a clockwise vortex is generated at the center of the obstacle and induces two counterclockwise rotating vortices. The vortices circumferentially separate the inward and outward flows along the fan's axis in a uniform manner, and their cores are circularly rotated by the clockwise vortex. These findings can contribute to the layout of fans under spatial restriction and suppression of flow instability due to obstacles.