Figure 8

Models for evolution of a hexamodular ancestral ferrichrome synthetase gene and for generation of domain architectures of the extant types examined in this study. A. Possible origin of a hexamodular ancestral ferrichrome synthetase gene. We propose that a hexamodular gene arose by a series of duplication events. Step one: module 1 duplicates, forming module 1 and new module 2. Step two: modules 1 and 2 duplicate together, forming modules 1 and 2, and new modules 3 and 4. Step three: modules 3 and 4 duplicate together, forming modules 3 and 4, and new modules 5 and 6. This scenario predicts that modules 1, 3, and 5 (dotted lines) will show greater similarity to each other than to other modules. Similarly, modules 2, 4, and 6 (solid lines) will show greater similarity to each other than to modules 1, 3 and 5. B. Possible scenarios generating members of the NPS2 and NPS1/SidC lineages from a hexamodular ancestral gene. Trees to the right show relationships of extant AMP domains, based on Fig. 3. Numbers in parentheses indicate corresponding domain of hypothetical ancestral gene. Left side of figure indicates proposed losses of A (black boxes) or C (white boxes) domains, resulting in the extant gene.