The two transducers in the phototaxis system of the archaeon Halobacterium salinarum, HtrI and HtrII, are methyl-accepting proteins homologous to the chemotaxis transducers in eubacteria. Consensus sequences predict three glutamate pairs containing potential methylation sites in HtrI and one in HtrII. Mutagenic substitution of an alanine pair for one of these, Glu265-Glu266, in HtrI and for the homologous Glu513-Glu514 in HtrII eliminated methylation of these two transducers, as demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis autofluorography. Photostimulation of the repellent receptor sensory rhodopsin II (SRII) induced reversible demethylation of HtrII, while no detectable change in the extent of methylation of HtrI was observed in response to stimulation of its cognate sensory rhodopsin, the attractant receptor SRI. Cells containing HtrI or HtrII with all consensus sites replaced by alanine still exhibited phototaxis responses and behavioral adaptation, and methanol release assays showed that methyl group turnover was still induced in response to photostimulation of SRI or SRII. By pulse-chase experiments with in vivo L-[methyl-³H]methionine-labeled cells, we found that repetitive photostimulation of SRI complexed with wild-type (or nonmethylatable) HtrI induced methyl group turnover in transducers other than HtrI to the same extent as in wild-type HtrI. Both attractant and repellent stimuli cause a transient increase in the turnover rate of methyl groups in wild-type H. salinarum cells. This result is unlike that obtained with Escherichia coli, in which attractant stimuli decrease and repellent stimuli increase turnover rate, and is similar to that obtained with Bacillus subtilis, which also shows turnover rate increases regardless of the nature of the stimulus. We found that a CheY deletion mutant of H. salinarum exhibited the E. coli-like asymmetric pattern, as has recently also been observed in B. subtilis. Further, we demonstrate that the CheY-dependent feedback effect does not require the stimulated transducer to be methylatable and operates globally on other transducers present in the cell.