The ability to respond to chemical stimuli is among the key sensory functions found in almost all living organ- isms. Green flagellate algae including Chlamydomonas that are model objects in many fields of biological research use chemical stimuli to adapt to their environ- ment by specific chemoinduced behavior. Although the first observations of such responses in Chlamydomonas date back to the 19th century [1], our knowledge of them is still at the level of phenomenology. On the contrary, significant progress has been achieved in investigation of chemoregulation of behavior in other cells. Best of all has been characterized chemotaxis in motile cells in humans and animals [2, 3], as well as chemotaxis in some prokaryotes (mainly enterobacteria [4, 5]). Also, much is known about chemotaxis in the lower eukaryotes: the myxamoeba Dictyostelium [6, 7] and the ciliate Paramecium [8, 9]. With the exception of Paramecium, success in inves- tigation of chemoregulation of motility in these particular objects was due to their being amenable for molecular genetic analyses. Recently completed sequencing of the whole nuclear genome of Chlamydomonas reinhardtii [10] created a possibility for rapid progress in research into chemoreception in this microorganism as well. This opti- mism is supported with the remarkable progress in inves- tigation of other physiological functions of this alga achieved since it became the object for establishment of nucleotide sequence databases. In particular, this facilitat- ed the identification of rhodopsin receptors for phototaxis and the photophobic response in C. reinhardtii [11, 12]. The nuclear genome of C. reinhardtii contains 100 to 110 million base pairs combined into 17 linkage groups (chromosomes). Its sequencing has been undertaken by the US Department of Energy Joint Genome Institute. By the time of this review, initial assembly of the infor- mation obtained by sequencing overlapping short frag- ments of the genome has been completed. The results of this work, as well as their preliminary analysis, can be accessed at the JGI web page: http://genome.jgi- psf.org/chlre2/chlre2.home. html. One of the reasons for Paramecium being a popular experimental organism is that it is suitable for electro- physiological measurements. It has been found that mechanisms for chemical signaling in Paramecium involve bioelectrical processes in the cell membrane, and detailed studies on these processes have been carried out [9]. Although Chlamydomonas is too small for reliable microelectrode recording widely used in Paramecium, methods for extracellular measurement of asymmetrical- ly localized transmembrane currents in this and related algae have been developed [13-15]. It has been shown that photoexcitation of rhodopsins leads to generation of such currents, which play a key role in photoregulation of motility in these microorganisms [16]. The possibility of photoelectric recording prompted investigation of pho- toreception and photomovement in Chlamydomonas [17]. Biochemistry (Moscow), Vol. 70, No. 7, 2005, pp. 717-725. Translated from Biokhimiya, Vol. 70, No. 7, 2005, pp. 869-879. Original Russian Text Copyright © 2005 by Govorunova, Sineshchekov. REVIEW 0006-2979/05/7007-0717 ©2005 Pleiades Publishing, Inc. * To whom correspondence should be addressed. Chemotaxis in the Green Flagellate Alga Chlamydomonas E. G. Govorunova* and O. A. Sineshchekov Faculty of Biology, Lomonosov Moscow State University, 119992 Moscow, Russia; fax: (7-095) 939-4309; E-mail: egovoru@yahoo.com Received July 14, 2004 Revision received August 12, 2004 Abstract—Behavior of the green flagellate alga Chlamydomonas changes in response to a number of chemical stimuli. Specific sensitivity of the cells to different substances might appear only at certain stages of the life cycle. The heterogamous species C. allensworthii demonstrates chemotaxis of male gametes towards pheromones excreted by female gametes. In C. reinhardtii chemotaxis towards tryptone occurs only in gametes, whereas chemotaxis towards ammonium, on the contrary, only in veg- etative cells. Chemotaxis to different chemical stimuli might involve different mechanisms of reception and signal transduc- tion, elucidation of which has only recently begun. Indirect evidences show that the cells likely respond to tryptone with changes in the membrane electrical conductance. The recently completed project of sequencing the whole nuclear genome of C. reinhardtii provides the basis for future identification of molecular elements of the chemosensory cascade in this alga. Key words: chemoreception, signal transduction, photoreceptor current, ammonium, tryptone, gametogenesis