Characterisation of two tomato fruit-expressed cDNAs encoding xyloglucan endo-transglycosylase.

Xyloglucan endo-transglycosylase (XET) catalyses the cleavage and concomitant transfer of one xyloglucan molecule to another. It is thought to be an important component of cell wall metabolism, particularly in expanding tissue and ripening fruits. The recently reported cloning of a cDNA encoding a seed-expressed XET from nasturtium [9] has enabled two XET-encoding cDNAs to be isolated from a tomato fruit (breaker stage) cDNA library. Their deduced amino acid sequences exhibit ca. 40% identity to nasturtium XET. One of the tomato cDNA clones (tXET-B2) was over-expressed in Escherichia coli; following purification and refolding, the recombinant protein was shown to have XET activity, with no detectable hydrolytic activity. Southern hybridisation analysis suggests that these clones are members of a small multi-gene family encoding tomato XET. Ribonuclease protection assays show that transcripts protected by one of the clones (tXET-B1) are most abundant in pink fruit pericarp and were also detected in stems.