Control of cell morphology of the yeast Saccharomyces cerevisiae by nutrient limitation in continuous culture

Saccharomyces cerevisiae IFO 0203, a polyploid yeast used in ethanol production in Japan, grows as ovoid cells in unstirred batch culture and on fully nutritive agar plates (2% w/v glucose; 0.67% w/v Difco yeast nitrogen base).
Extensively branched pseudohyphae formed on 0.01% w/v ammonium sulphate plates within a few days. In continuous culture with high oxygen supply and limiting glucose, cells were elongated but growth was vigorous and the daughter cells separated well after budding.
Limitation of growth by either nitrogen source or oxygen during continuous culture resulted in formation of truncated, occasionally branched, pseudohyphae up to five cells in length.     

CDNA cloning of p112, the largest regulatory subunit of the human 26s proteasome, and functional analysis of its yeast homologue, sen3p.

The 26S proteasome is a large multisubunit protease complex, the largest regulatory subunit of which is a component named p112. Molecular cloning of cDNA encoding human p112 revealed a polypeptide predicted to have 953 amino acid residues and a molecular mass of 105,865. The human p112 gene was mapped to the q37.1-q37.2 region of chromosome 2. Computer analysis showed that p112 has strong similarity to the Saccharomyces cerevisiae Sen3p, which has been listed in a gene bank as a factor affecting tRNA splicing endonuclease. The SEN3 also was identified in a synthetic lethal screen with the nin1-1 mutant, a temperature-sensitive mutant of NIN1. NIN1 encodes p31, another regulatory subunit of the 26S proteasome, which is necessary for activation of Cdc28p kinase. Disruption of the SEN3 did not affect cell viability, but led to temperature-sensitive growth. The human p112 cDNA suppressed the growth defect at high temperature in a SEN3 disruptant, indicating that p112 is a functional homologue of the yeast Sen3p. Maintenance of SEN3 disruptant cells at the restrictive temperature resulted in a variety of cellular dysfunctions, including defects in proteolysis mediated by the ubiquitin pathway, in the N-end rule system, in the stress response upon cadmium exposure, and in nuclear protein transportation. The functional abnormality induced by SEN3 disruption differs considerably from various phenotypes shown by the nin1-1 mutation, suggesting that these two regulatory subunits of the 26S proteasome play distinct roles in the various processes mediated by the 26S proteasome.     

Regulation of Homocysteine Biosynthesis in Salmonella typhimurium

The regulation of the homocysteine branch of the methionine biosynthetic pathway in Salmonella typhimurium has been reexamined with the aid of a new assay for the first enzyme. The activity of this enzyme is subject to synergistic feedback inhibition by methionine plus S-adenosylmethionine. The synthesis of all three enzymes of the pathway is regulated by noncoordinate repression. The enzymes are derepressed in metJ and metK regulatory mutants, suggesting the existence of regulatory elements common to all three. Experiments with a methionine/vitamin B(12) auxotroph (metE) grown in a chemostat on methionine or vitamin B(12) suggested that the first enzyme is more sensitive to repression by methionine derived from exogenous than from endogenous sources. metB and metC mutants grown on methionine in the chemostat did not show hypersensitivity to repression by exogenous methionine. Therefore, it appears that the metE chemostat findings are peculiar to the phenotype of this mutant; such evidence suggests a possible role for a functional methyltetrahydrofolate-homocysteine transmethylase in regulating the synthesis of the first enzyme. Thus there appear to be regulatory elements which are common to the repression of all three enzymes, as well as some that are unique to the first enzyme. The nature of the corepressor is not known, but it may be a derivative of S-adenosylmethionine. metJ and metK mutants of Salmonella have a normal capacity for S-adenosylmethionine synthesis but may be blocked in synthesis or utilization of a corepressor derived from it.     

Lidocaine block of cardiac sodium channels

Lidocaine block of cardiac sodium channels was studied in voltage-clamped rabbit purkinje fibers at drug concentrations ranging from 1 mM down to effective antiarrhythmic doses (5-20 μM). Dose-response curves indicated that lidocaine blocks the channel by binding one-to-one, with a voltage-dependent K(d). The half-blocking concentration varied from more than 300 μM, at a negative holding potential where inactivation was completely removed, to approximately 10 μM, at a depolarized holding potential where inactivation was nearly complete. Lidocaine block showed prominent use dependence with trains of depolarizing pulses from a negative holding potential. During the interval between pulses, repriming of I (Na) displayed two exponential components, a normally recovering component (τless than 0.2 s), and a lidocaine-induced, slowly recovering fraction (τ approximately 1-2 s at pH 7.0). Raising the lidocaine concentration magnified the slowly recovering fraction without changing its time course; after a long depolarization, this fraction was one-half at approximately 10 μM lidocaine, just as expected if it corresponded to drug-bound, inactivated channels. At less than or equal to 20 μM lidocaine, the slowly recovering fraction grew exponentially to a steady level as the preceding depolarization was prolonged; the time course was the same for strong or weak depolarizations, that is, with or without significant activation of I(Na). This argues that use dependence at therapeutic levels reflects block of inactivated channels, rather than block of open channels. Overall, these results provide direct evidence for the “modulated-receptor hypothesis” of Hille (1977) and Hondeghem and Katzung (1977). Unlike tetrodotoxin, lidocaine shows similar interactions with Na channels of heart, nerve, and skeletal muscle.     

Hypoxanthine and thymidine compete for transport in Chinese hamster fibroblasts

The transport of thymidine and hypoxanthine was investigated in mutant Chinese hamster lung fibroblasts deficient in both thymidine kinase and hypoxanthine-guanine phosphoribosyltransferase. Kinetic data from rapid uptake experiments (0.5–4.5 s) indicate that thymidine is transported by a monophasic saturable system (K_m = 0.29 mM, V = 6.7 nmol/min · mg) which is competitively inhibited by hypoxanthine (K_i = 3.3 mM). The cells displayed a single transport system for hypoxanthine (K_m = 2.0 mM, V = 8.9 nmol/min · mg) that is inhibited competitively by thymidine (K_i = 0.43 mM). Both hypoxanthine and thymidine entry were noncompetively inhibited by nitrobenzylthioinosine, but thymidine transport was more sensitive. A kinetic model in which hypoxanthine and thymidine share a common transporter can account for the competitive inhibition and the observation that the inhibition constants are similar to the Michaelis constants.     

Polyclonal antisera specific for the proenzyme form of each calpain.

Each subunit of calpain (EC 3.4.22.17) is proteolytically modified when the enzymes are exposed to calcium. These cleavages appear to be important for regulating the proteolytic activity and calcium-sensitivity of the proteinases. We have synthesized peptides that correspond to the sites of autoproteolytic modification within the catalytic subunit of each calpain. Polyclonal antisera raised against these peptides are highly specific for the unmodified catalytic subunit of each calpain. The antiserum specific for the N-terminal epitope of milli-calpain was used to demonstrate an inverse relationship between the presence of this N-terminal peptide and casein hydrolysis. The antiserum specific for the N-terminal epitope of micro-calpain was used to demonstrate proteolytic modification of the catalytic subunit of mu-calpain in rat erythrocytes treated with ionomycin and calcium.     

A microtiter plate transglutaminase assay utilizing 5-(biotinamido)pentylamine as substrate.

Transglutaminases belong to an important family of enzymes involved in hemostasis, skin formation, and wound healing. We describe a technique for the measurement of transglutaminase activity using polystyrene microtiter plates coated with N,N'-dimethylcasein. The substrate 5-(biotinamido)pentylamine is covalently incorporated into N,N'-dimethylcasein by transglutaminase in a calcium-dependent reaction. The biotinylated product is detected by streptavidin-alkaline phosphatase and quantitated by measuring the absorbance at 405 nm following the addition of p-nitrophenyl phosphate. The assay is sensitive, specific, and linear at plasma factor XIIIa concentrations between 0.08 and 1.25 micrograms/ml and at purified guinea pig liver transglutaminase concentrations between 0.05 and 0.8 microgram/ml. The intra-assay coefficient of variation is less than 8%. The solid-phase assay was used to quantitate the transglutaminase activity in Escherichia coli extracts expressing recombinant factor XIII A-chains and to analyze factor XIIIa inhibitors. This method will facilitate the analysis of structure-function relationships of the transglutaminases using recombinant DNA methods. Furthermore, screening of natural and synthetic factor XIIIa inhibitors will be expedited by this solid-phase microtiter plate assay.