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The strategy for detecting oxygen, carbon monoxide, nitric oxide, and sulfides is predominantly through heme-based sensors utilizing either a globin domain or a PAS domain. Whereas PAS domains bind various cofactors, globins bind only heme. Globin-coupled sensors (GCSs) couple an N-terminal sensor globin domain to varied C-terminal signaling domains to effect aerotaxis and gene regulation. Having descended from an ancient protoglobin, GCSs are now ubiquitous and are encoded in the genome of several extremophiles (temperature, salt, and pH). We postulate that their role in regulating gene expression governs microbial processes critical to elemental recycling, bioremediation, and cellulose degradation. Functional and evolutionary analyses of the GCSs, their protoglobin ancestor, and their relationship to the Last Universal Common Ancestor (LUCA) are also discussed in the context of globin-based signal transduction.

Globin-coupled sensors (GCSs) are multidomain proteins, consisting of an N-terminal globin domain fused to a variety of C-terminal transmitter domains. Functional classification of GCSs is based on the transmitter domain(s) they possess, broadly falling under either aerotaxis or gene regulation. This chapter describes methods and strategies for cloning, expression, and purification of GCSs for spectroscopic analysis and determination of the minimum heme-binding domain, together with bioinformatic approaches for database searching and examination of domain architectures.