Organization of the frp operons in S. aureus (A), B. halodurans (B), and L. monocytogenes (C). The open reading frames are predicted to encode proteins as follows: A, 40-kDa cell wall protein homologue; B, 87-kDa cell wall protein homologue; C and D, membrane proteins; E, siderophore binding protein; F, transport permease protein; X, ATP binding protein; G, membrane protein; H, hypothetical protein.