FIG. 4.

Biosynthesis of the cyclic bacterial proteins MccJ25 and AS-48. (a) Four genes located within the mcj cluster, mcjABCD, are necessary for the production of and immunity to MccJ25. Maturation from the precursor McjA to MccJ25 requires both McjB and McjC (blue) and, due to the predicted absence of membrane-spanning domains in these proteins, probably occurs in the cytosol. McjD (aqua), which confers immunity to MccJ25, shows homology to bacterial ABC exporters and has been implicated in secretion of MccJ25 from the cell. A chromosomally encoded outer membrane protein, TolC (yellow), has since also been found to play a role in MccJ25 secretion (18). (b) Ten genes implicated in AS-48 production and immunity are located within the as-48 gene cluster, as-48ABCC1DD1EFGH, on the pMB2 plasmid. The gene products As-48ABCC1DD1 are necessary for AS-48 production and immunity and, except for the precursor protein As-48A, are all predicted to be located in the membrane. As-48B and As-48C (blue) are predominantly involved in AS-48 production. Their putative locations in the membrane may indicate that AS-48 processing and cyclization are coupled to secretion, as shown here. Alone, As-48D1 (red) is able to confer immunity on the cyclic protein AS-48, but As-48C1 and As-48D (yellow), the latter showing homology to ABC transporter proteins, enhance this resistance, and thus all three proteins have been implicated in AS-48 secretion. The as-49EFGH operon encodes another putative ABC transporter (aqua) which was recently shown to be required for full expression of AS-48 immunity. The precursor proteins are represented by purple strands, with the cyclic protein domains colored orange. IM, inner membrane; OM, outer membrane.