Cellulose synthase (CesA) genes in algae and seedless plants
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Microfibril structure is determined largely by the organization of arrays of integral plasma membrane protein particles known as “terminal complexes”, which include cellulose synthase catalytic subunits encoded by CesA genes. Although the CesA genes of plants and bacteria share conserved regions, variations in terminal complex and microfibril structure presumably result from sequence differences. Thus, the CesA domains that influence terminal complex assembly may be revealed by examining the differences between CesA genes from green algae in which terminal complex structure ranges from rosettes (plant-like) to linear (bacteria-like). This report describes a second CesA gene that has been cloned from Mesotaenium caldariorum, a unicellular green alga from the order Zygnematales, which have rosette terminal complexes. Both McCesA1 and McCesA2 are similar to seed plant CesAs in domain structure and intron position. Seed plants have multiple CesAs and CesA-like (Csl) genes, some of which appear to be expressed specifically during cell expansion, secondary cell wall deposition in vascular tissue, or tip growth. Diversification of the CesA and Csl gene families can be explored by comparing these genes in mosses, which lack vascular tissue with secondary cell walls, and early divergent vascular plants such as ferns. Degenerate primers were used to amplify and clone five unique CesA and Csl fragments from genomic DNA isolated from Physcomitrella patens. Probes derived from the cloned fragments were used to isolate several clones from a Physcomitrella genomic library. One Csl fragment was amplified from genomic DNA isolated from the fern Ceratopteris richardii. Phylogenetic analysis supports the presence of CslD genes in both mosses and ferns, but does not support the presence of secondary cell wall specific CesA orthologs in mosses.
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