An Mrp-Like Cluster in the Halotolerant Cyanobacterium

The mrp homolog gene cluster mrpCD1D2EFGAB (Ap-mrp) was found in a halotolerant cyanobacterium, Aphanothece halophytica, amplified, and expressed in Escherichia colimutant TO114. Ap-mrp complemented the salt-sensitive phenotype of TO114 and exhibited Na/H and Li/H exchange activities, indicating that Ap-Mrp functions as a Na/H antiporter. Bacterial mrp (multiple resistance and pH adaptation) loci encode highly unusual multisubunit cation/proton antiporters (CPAs) (13). Mrp systems are widespread among bacteria and archaea and, because of their unique complexity, are classified as the separate CPA3 family (12). They are known as group 1, group 2, and others (mrp-like gene clusters) (13). Group 1 and group 2 Mrp antiporters have been shown to have roles in alkaline pH homeostasis and Na resistance (5, 8, 9), sporu-lation (9), symbiotic nitrogen fixation (11), pathogenesis (10), arsenite resistance (8), and bile salt resistance (7). Cyanobac-terial Mrp-like clusters have been reported to be involved i