Eukaryotic cellulose synthases

Available evidence supports a common ancestry for all cellulose synthases. These enzymes appear to have been a bacterial invention acquired by various eukaryotes via multiple lateral gene transfers. However, the proteins associated with regulation of cellulose biosynthesis and polymer crystallization seem to have evolved independently. Sequence divergence of eukaryotic cellulose synthases and the presence of multiple gene clusters associated with bacterial cellulose synthases are discussed in relation to the possible evolutionary pathways of cellulose biosynthesis. 

Table 3-2. Protein model and conserved motifs for the rosette-forming eukaryotic cellulose synthase (CESA)...

Table 1-1. Expectation values from pairwise BLAST alignments of eukaryotic and prokaryotic cellulose synthases...

These observations suggest a lateral transfer of cellulose synthase from cyanobacteria to D. discoideum. However, while the primary and secondary endo-symbiotic events that led to the evolution of plastids in plants and algae provide a clear mechanism for the transfer of a cyanobacterial cellulose synthase to photosynthetic organisms, such a mechanism is lacking for D. discoideum. Cyanobacterial genes are known to exist in eukaryotes which have secondarily lost plastids. However, there is no evidence for the existence of an endosymbiotic relationship between ancestors of D. discoideum and a cyanobacterium. Therefore, if a lateral transfer occurred, it was likely xenologous, possibly via a food ratchet mechanism (Doolittle 1998). 

The proliferation of complete genome sequences in public databases provides an additional means to track the evolution of cellulose in bacteria. Conservation of operons and/or gene clusters (synteny) can be used to trace not only the history of cellulose synthase, but also its associated proteins. The existence of a few these gene clusters has been well documented. In the sections below, I would like to give a brief review of the known gene organizations and introduce two novel ones which may be linked to the eukaryotic acquisition of cellulose biosynthesis. 

Current data suggest that cellulose biosynthesis is a bacterial invention and that eukaryotes acquired the process via multiple lateral gene transfers. Bacteria and eukaryota have independently evolved regulatory mechanisms and molecular structures to utilize the p-1,4-homopolymer synthesized by the catalytic activity of homologous cellulose synthase enzymes. The differences in accessory enzymes probably reflect not only convergent evolution to produce a cellulose I crystalline allomorph, but also inventions of alternative products such as cellulose II, noncrystalline cellulose, or nematic ordered cellulose. 

Roberts A.W., Roberts E.M., and Delmer D.P. 2002. Cellulose synthase (CesA) genes in the green alga Mesotaenium caldariorttm. Eukaryotic Cell 1 847-855. 

Conserved U motifs originally identified in bacterial cellulose synthases were used to identify the higher plant enzymes, which also contain a conserved zinc-binding domain (ZnBD) specific to the eukaryotic enzymes (Saxena et al. 1995). 

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