Cellulose synthase genes were first identified in A. xylinum
and subsequently in
other bacterial species (Matthysse et al.
, 1995b; Saxena et al.
, 1990; Wong et al.
1990) before they were identified in plants (Arioli et al.
, 1998; Pear et al.
, 1996). A. xylinum
produces abundant amounts of cellulose, and it has been a model
organism for studies on cellulose biosynthesis, so it is not surprising that cellulose
biosynthesis genes were first identified in this organism. Interestingly, the genes
from this organism were not found to be useful in isolating cellulose synthase
genes from other organisms by nucleic acid hybridization techniques. However,
Saxena et al.
(1995) compared the derived amino acid sequence of the bacterial cellulose synthase with other proteins and found them useful in identifying
conserved amino acid residues in β-glycosyltransferases, more specifically the
conserved residues and sequence motif identified as D, D, D, QXXRW in processive
β-glycosyltransferases (Saxena et al.
, 1995). Based on the deduced amino acid
sequences of bacterial cellulose synthases and other β-glycosyltransferases, genes
for plant cellulose synthases were first identified by random sequencing of a cotton
fiber cDNA library (Pear et al.
, 1996). Two cDNA clones (GhCesA1
were identified from the cotton fiber cDNA library, and the derived amino acid
sequence of GhCesA1
gave the first glimpse of the primary structure of a plant
cellulose synthase (Pear et al.
, 1996). In addition to the transmembrane regions and
the conserved residues found in bacterial cellulose synthase, the cellulose synthase
from plants was found to have additional features—the presence of two regions
(originally referred to as CR-P and HVR) within the globular domain that
contained the conserved residues and a zinc-finger domain at the N-terminus.
Around the same time that cDNA clones encoding cellulose synthases were
identified in cotton by random sequencing (Pear et al.
, 1996), a number of cDNA
clones encoding amino acid sequences containing the D, D, D, QXXRW conserved
residues and sequence motif were identified by sequence analysis of expressed
sequence tag (EST) sequences of Arabidopsis
and rice that were available in the
public databases (Cutler and Somerville, 1997; Saxena and Brown, 1997). However,
the proteins encoded by these cDNA clones did not show the additional
features identified in the cotton cellulose synthases; instead these proteins resembled
more the primary structure of the bacterial cellulose synthase and were
referred to as cellulose synthase-like proteins with a role possibly in the synthesis
of β-linked polysaccharides other than cellulose (Cutler and Somerville, 1997).
Soon thereafter, a superfamily of genes encoding cellulose synthases (CesA
cellulose synthase-like (Csl
) proteins were identified in a large number of plants
(Richmond and Somerville, 2000). The presence of a large number of genes
belonging to the cellulose synthase superfamily in each plant was surprising at
first, but the role of many of these CesA
genes in cellulose biosynthesis became
obvious following analyses of a number of Arabidopsis
mutants affected in cellulose
biosynthesis. Interestingly, two cellulose synthase genes were earlier identified
in A. xylinum
(Saxena and Brown, 1995). Although both genes encode a
functional cellulose synthase as determined by in vitro
cellulose synthase activities
in mutants, only one gene was found to be essential for normal in vivo
synthesis in A. xylinum
(Saxena and Brown, 1995).