Carotenoid biosynthesis in bacteria: the crt gene products and their functional roles in the carotenogenic pathways

Carotenoids are isoprenoid pigments that are largely responsible for the red, pink, orange, and yellow pigmentation in bacteria. Despite their structural diversity, they share a similar general chemical structure. Carotenogenesis is a complex, multistep process, mediated by the crt gene products. The crt genes encode enzymes that catalyze a wide array of reactions within the carotenogenic pathways, sometimes showcasing broad substrate specificity. These enzymes are involved in processes such as condensation, desaturation, oxygenation, cyclization, hydroxylation, ketolation, glycosylation, acylation, elongation, and methylation of carotenoid intermediates. Some crt genes do not encode enzymes, but rather regulators of carotenogenesis. This review provides an in-depth exploration of the multitude of crt genes identified in various bacteria, emphasizing the pivotal role of Crt enzymes, their diverse functions within the different carotenogenic pathways and some of the reactions they catalyze. Additionally, the biosynthetic pathways of C30, C40, C45, and C50 carotenoids, as well as the production of certain rare carotenoids in bacteria, are explored. Overall, this review highlights the importance of crt gene products in the diverse and tightly regulated biosynthesis pathways of bacterial carotenoids.