ISSN 0026-2617, Microbiology, 2010, Vol. 79, No. 2, pp. 153–159. © Pleiades Publishing, Ltd., 2010. Original Russian Text © I.V. Fedoseeva, N.N. Varakina, T.M. Rusaleva, G.B. Borovskii, E.G. Rikhvanov, V.K. Voinikov, 2010, published in Mikrobiologiya, 2010, Vol. 79, No. 2, pp. 173–179. 153 Under optimal conditions, the level of calcium in the cytosol of yeast cells is 100–200 nM. Maintaining the low calcium concentration is extremely important for normal cell functions. A short-term increase in Ca 2+ concentration under various physiological stim- uli is necessary for the regulation of Ca 2+ -dependent enzymes, for the cell growth cycle and activation of the stress genes expression. Long-term incubation with the sexual pheromone (α factor), hypotonic shock, addition of glucose to the starving cells, amiodarone treatment, etc., were shown to cause an increase of calcium concentration in the yeast cytoplasm. This increase may due either to calcium inflow from the external medium or to its release from the inner stores such as vacuole and endoplasmic reticulum [1]. Long- term increase of calcium level in the cytosol may lead to programmed cell death (PCD) [2]. The microarray method was used to demonstrate that incubation of yeast cells with calcium induced the expression of more than 160 genes [3]. The proteins encoded by these genes participate in the regulation of a set of metabolic pathways and in the transport of ions or small molecules that regulate the ion homeosta- sis:PMC1 coding for Ca 2+ ATPase responsible for Ca 2+ transport of into the vacuole; PMR1 coding for the car- rier of Ca 2+ and Mn 2+ to the Golgi apparatus; ENA1 coding for Na + /Li ATPase from the cytoplasmic mem- brane, etc. Transcription of these genes is necessary for the growth of yeast at high concentrations of calcium in the medium. Their expression is controlled by Ca 2+ /calmodulin-dependent phosphatase (cal- cineurin) [1] participating in activation of the Crz1 transcriptional factor [1, 3]. Synthesis of the heat shock proteins (HSPs) is induced under heat shock thus promoting the devel- opment of induced heat tolerance to the subsequent lethal heat treatment. The role of the Hsp104 protein from S. cerevisiae in the process is well enough studied. In yeast cells, Hsp104 causes dissociation of the pro- teins aggregated by the heat shock. This results in enhanced cell viability after the lethal heat shock [4]. Transcription of the heat shock genes in eukaryotes is regulated by binding of the heat shock transcriptional factor (Hsf) to the heat shock elements (HSE) located in promoters of the corresponding genes [5]. In S. cer- evisiae, heat shock and other stresses activate an Hsf- independent mechanism for activation of HSP genes' expression. Two highly homologous transcriptional factors, Msn2 and Msn4, participate in this process via association with the stress-sensitive element STRE [6]. The nature of the signal activating transcriptional factors involved in the heat shock response is still unknown. Denaturation of intracellular proteins [7] and intensified generation of reactive oxygen species (ROS) are suggested to play the role of a signal [8]. Fluctuations in the level of intracellular calcium con- centration in plant [9] and animal cells [5, 10] under heat shock are also considered as possible signals affecting Hsf transcriptional activity. Heat shock in plant and animal cells was found to cause an increase of calcium concentration in the cytosol followed by Effect of Calcium Ions on the Hsp104 Synthesis and Heat Tolerance of Saccharomyces cerevisiae I. V. Fedoseeva 1 , N. N. Varakina, T. M. Rusaleva, G. B. Borovskii, E. G. Rikhvanov, and V. K. Voinikov Siberian Institute of Plant Physiology and Biochemistry, Russian Academy of Sciences, Irkutsk, Russia Received April 2, 2009 Abstract—Effect of calcium ions on heat tolerance of Saccharomyces cerevisiae and on the induction of Hsp104 synthesis by this microorganism was studied. Short-term (30 min) treatment with CaCl 2 at 30°C enhanced the heat tolerance to the lethal heat shock (50°C); the synthesis of Hsp104 was induced as well. The effect of Ca 2+ on the heat tolerance and Hsp104 synthesis was shown to be ion-specific and was inhibited by LaCl 3 , which is known to block calcium ion channels on the cytoplasmic membrane. The effect of Ca 2+ depended on the potential of the inner mitochondrial membrane. When the cells were treated with sodium azide, which reduced the electrochemical potential, the effect of calcium both on heat tolerance and Hsp104 synthesis was suppressed. Depending on the concentration of exogenous Ca 2+ and the ambient conditions, calcium ions may either induce or inhibit the expression of the stress genes and cell viability. Key words: Saccharomyces cerevisiae, heat tolerance, calcium, Hsp104, mitochondria. DOI: 10.1134/S0026261710020049 EXPERIMENTAL ARTICLES 1 Corresponding author; e-mail: fedoseeva@sifibr.irk.ru