Rare metabolic gene essentiality is a determinant of microniche adaptation in Eschherichia coli.

Rare genes in bacterial pangenomes have historically been considered non-essential, dispensable, or even costly, and largely excluded from in-depth analyses due to their perceived redundancy, high variability, and presumed neutral evolutionary origin. However, whether rare genes contribute to metabolic robustness when core genes are lost remains an open question. In this study, we systematically investigate the role of rare metabolic genes in Escherichia coli, revealing their essentiality in maintaining metabolic functions under core gene loss. Through a pangenome-scale reconstruction of 15311 strain-specific genome-scale models (panGEM) and over 22.4 million gene knockout simulations, we demonstrate that: (i) 9.4% of rare metabolic genes are essential in at least one of three key host environments-feces, serum, and urine; (ii) 41% of strains rely on at least one rare essential metabolic gene for survival; (iii) rare metabolic genes emerge as a result of microniche adaptation, and (iv) panGEM allows for the prediction of a subset of highly conserved metabolic reactions with minimal genetic diversity as stable drug targets. These findings challenge the common view that rare genes primarily serve as evolutionary byproducts of genome fluidity and reveal their critical role in metabolic resilience.