R161, K452 and R460 residues are vital for metal-citrate complex transport in Cit_Sc from streptomyces coelicolor

Recent discoveries have been made that demonstrate Gram-positive bacteria can transport metal-citrate complexes through the CitMHS family of proteins in symport with H⁺ ions. The CitMHS family of transporters investigated to date have the ability to selectively transport only certain metal-citrate complexes. Despite sharing amino acid sequence similarity as high as 73%; predicting what complexes are transported remains difficult. The iron-citrate transporter from Streptomyces coelicolor has been mutated at three postulated critical sites (R161, K452 and R460) based on activity modeling against the LacY permease. All three mutants eliminate or greatly reduce uptake of metal-citrate complexes tested. The implications of this are discussed.