Istituto Sperimentale per la Cerealicoltura
Max-Planck-Institut für Züchtungsforschung
The expression of the gene was analysed on the RNA and protein level. The Gl2 gene is transcribed in young leaves, in coleoptiles, in husks, in immature ears and in silks of wild type plants. No transcripts were found in roots of wild type plants or in tissues of plants homozygous for a recessive gl2-ref allele. Antibodies against the GL2 protein revealed predominant presence in the leaves with a juvenile wax phenotype. Protein detection was consistent with transcriptional activity of the gene in wild type coleoptiles, husks, immature ears and silks but not in roots, anthers and seeds. Sequence data bank analysis revealed homologies of the GL2 protein of maize with eighteen sequences from several plant species.
A domain of 27 amino acids is highly conserved in the proteins of different plant species. This domain, PLx3QxTxFxCGGx8Hx3D, is novel and with a function still unknown. Interestingly, the domain is present in the four proteins found in Clarika breweri, Cantharanthus roseus, Gentiana triflora and Dianthus caryophyllus which are credited to have a coenzymeA-dependent acyl transferase activity. Based on this finding, already St. Pierre et al. (Plant J. 14: 703-713, 1998) proposed that the GL2 protein belongs to plant proteins which function as acetyl transferases, an enzymatic activity which has still to be demonstrated for gl2.
For the Gl2 gene the highest
sequence similarity was observed with the Cer2 Arabidopsis
gene, having both gene mutant alleles conditioning a defect in wax biosynthesis
(Tacke et al., 1995; Negruk et al., Plant J. 9: 137-145, 1996). Their mutant
waxes are defective in the accumulation of C30 and C32 fatty acids, respectively.
The fact that both mutants are defective in the last step of fatty acid
elongation might indeed support a role of Cer-2 and Gl2 genes
in acyl-CoA-dependent fatty acid biosynthesis. This putative role of Gl2
and Cer-2 as structural genes involved in wax biosynthesis,
integrates the current knowledge on this process based on molecular studies
of the Gl1 and Gl8 maize loci. These genes exhibit sequence
similarities to other plant wax synthetic genes coding a putative transfer
protein (Hansen et al., Plant Physiol. 113: 1091-1100, 1997) and a ß-ketoacyl
reductase (Xu et al., Plant Physiol. 115: 507-510, 1997). It is interesting
to note that all the putative proteins with homology to the GL2 protein
contain from 420 to 470 amino acids. This detail may indicate their common
origin from a precursor gene, which later on evolved acquiring specific
functions in different biochemical pathways.
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