Aerotaxis, previously studied in eubacteria was investigated in the archaebacterium Halobacterium halobium. A quantitative temporal assay for aerotaxis of H. halobium was developed using computer assisted motion analysis. Aerotaxis was most pronounced in early log phase cultures. Peak aerotaxis was found to coincide with peak respiration consistent with a model in which respiration is required for aerotaxis. In a spatial assay a mutant strain lacking the proton motive force (pmf)-generating pigments, bacteriorhodopsin (bR) and halorhodopsin (hR), exhibited enhanced aerotaxis bands relative to the bR+hR+ parent. The enhanced aerotaxis in the mutant cells was not due solely to an enhanced respiration rate. Aerotaxis in the bR+hR+ strain was stronger when cells were incubated in the dark than in illuminated cells in which proton motive force was enhanced by the light-activated pigments. These results suggest that in H. halobium, as in eubacteria, aerotaxis is mediated by oxygen-dependent changes in proton motive force. In H. halobium S9P, methionine starvation depleted S-adenosylmethionine (as shown by high performance liquid chromatography) thereby inhibiting protein methylation. Unexpectedly, aerotaxis was abolished by methionine starvation even though unstimulated behavior was unchanged. Using a modified flow assay to measure protein methylesterase activity in vivo, peaks of esterase activity were observed following positive and negative oxygen stimuli and these changes were absent in a protein methyltransferase mutant. The pattern of altered methylesterase activity in H. halobium was most similar to chemostimulus related changes seen in Bacillus subtilis. H. halobium thus appears to be the first bacterium in which aerotaxis has been shown to be dependent on receptor methylation.
Biochemistry, Microbiology, Halobacterium; Methylation; Transducers; Chemotaxis; Ethionine; Methionine
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