Histidine-rich proteins in prokaryotes: Metal homeostasis and environmental habitat-related occurrence

Increasing amounts of histidine-rich proteins (HRPs) have been found in microorganisms. We systematically analyzed the proteomes of 675 prokaryotes including 52 archaea and 623 bacteria for histidine-rich motifs (HRMs). We show that HRPs are widespread in prokaryotic proteomes, with the majority being involved in metal homeostasis. HRPs are frequently found in the proteomes of certain orders of rhizobia and pathogenic Gram-negative bacteria, but are essentially absent in obligate intracellular pathogenic species. The occurrence of HRPs in the proteomes of prokaryotes is related to their habitats. We further revealed a class of globally histidine-rich bacterial proteins. This approach can readily be used to identify other single amino acid rich motifs (and proteins) in microbial proteomes to facilitate the exploration of their functions.