Fig. 1.
Evolution and instances of “frozen (metabolic) accidents.” a The standard genetic code (SGC) is a frozen genetic accident and became universal during early prokaryote evolution because of the eminent role of horizontal gene transfer (HGT) (left side). Each circle represents a species and different genetic codes are indicated by different fillings. Species with rare codes became extinct because they were less amenable to stable HGT. The code that remained became the SGC. Once it was universally adopted, it persisted even when HGT became less important. On the right, the evolution of photosystem subunits is schematically shown as an example of a FMA. In the thylakoid membrane (yellow), the core photosystem subunits (circles), having more than one transmembrane domain (TM), multiply interact with each other (dotted lines) and with their co-factors. They also interact to a lesser extent with other transmembrane photosystem proteins (with one TM, rectangles), as well as with peripheral photosystem proteins and soluble proteins (triangles). This leads to different rates and modes of evolution and sequence diversification (highlighted in gray, with increased variation being indicated by darker shading). The sequences of photosystem core subunits changed least during evolution and represent FMAs. b RuBisCO, PSII, and the molybdenum (Mo)-dependent cyanobacterial nitrogenase. RuBisCO consists of eight large (light green) and eight small (dark green) subunits and catalyzes carboxylation of ribulose-1,5-bisphosphate during the Calvin cycle. PSII contains a core with several subunits, of which D1 and D2 are shown. Other PSII subunits, including the oxygen-evolving complex (OEC), surround the core. In cyanobacteria, phycobilisomes act as antenna complexes, whereas light-harvesting complex II (LHCII) serves this function in eukaryotes. The Mo-nitrogenase is a two-component system comprising a heterotetrameric dinitrogenase complex with a P-cluster (P; Fe8S7) and an Fe-Mo cofactor (M), and a homodimeric dinitrogenase reductase with a Fe4S4 cluster (F)