Interrelationship between Ca 2+ and a methionine‐requiring step in Halobacterium halobium taxis

Halobacterium halobium possesses both phototaxis [1] and chemotaxis towards (or away from) attractants and repellants [2,3]. According to our data, the positive phototaxis towards green light which is mediated by the bacteriorhodopsin H + pump [1,4,5] is governed by A~ H+-sensing [6]. Blue light evokes negative phototaxis of H. halobium cells, and it was suggested that an as yet uncharacterized retinal-requiring pigment P37o [1,4,5] acts by causing a depolarization of the membrane [4,5]. However, we have recently found that blue light does not change the energy level at intensities that repel bacteria and suggested that PaT0 is a specific photoreceptor [6]. Sensory information from individual bacterial receptors is passed to methyl-accepting chemotaxis proteins (MCP) [7]; their methylation causes adaptation to attractant [8,9]. Recently a discovery of an MCP protein in H. halobium was reported [10]. It was shown that the addition of attractants increased, while the addition of a repellant decreased, -CH3-incorporation into a protein fraction. L-Ethionine, a non-metabolizing analogue of methionine, blocks methylation and was found to inhibit positive chemotaxis, but not phototaxis, in H. halobium [2]. A methyl-requiring process was also reported to be non-essential in the sensing of uncouplers by Bacillus subtilis [11]. We consider the sensing of uncouplers to be mediated by Ag H + reception and thus to be closely related to H. halobium positive phototaxis. We therefore attempted to re-investigate the methionine requirement in H. halobiurn phototaxis. In this paper we report that both positive and negative phototaxis in H. halobium is dependent upon methylation, and that a next step in the information processing requires Ca 2+ .