| Summary: | An empirical observation suggests that Giardia lamblia proteins have larger cysteine content than their counterparts in other organisms. As this parasite lacks conventional antioxidant stress systems, it is generally accepted that high cysteine content helps G. lamblia cope with oxygen toxicity, a strategy apparently shared by other organisms. Here, we question whether the high cysteine content in some organisms is genuine or just a simple assumption based on singular observations. To this end, we analyzed the cysteine content in 78 proteomes of organisms spanning the three domains of life. The results indicate that the cysteine content in eukaryota is approximately double that in archaea and bacteria, with G. lamblia among the highest. Atypical cysteine contents were found in a few organisms correlating with specific environmental conditions, supporting the evolutionary amino acid-level selection of amino acid composition.
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