The tomato hexokinase LeHXK1 cloning, mapping, expression pattern and phylogenetic relationships

Abstract

A tomato ( Lycopersicon esculentum L. Mill.) hexokinase, LeHXK1, was cloned from a tomato fruit cDNA library. Functional complementation of triple-mutant yeast cells ( hxk1 hxk2 glk1) that lack the ability to phosphorylate glucose and fructose confirmed the identity of LeHXK1 as a hexokinase. LeHXK1 was mapped to tomato chromosome 3-distinct from LeHXK2 which mapped to chromosome 6. Interestingly, expression of LeHXK1 was very low in leaves and very high in young tomato fruits. Two hexokinase isozymes, HXKl and HXK2, were chromatographically separated from young fruit extracts, Antisense StHXK1 the homologue of, LeHXK1, knocked out the HXKl isozyme, identifying HXKl as the gene product of LeHXK1. Phylogenetic analysis of eukaryotic and prokaryotic hexose phosphorylating enzymes indicates that tomato hexokinase genes, LeHXK1 and LeHXK2, and the known eukaryotic hexokinases and ‘glucokinases’ belong to the same hexokinase family whose members are unrelated to the glucose-specific glucokinases of bacteria. Hence, the data suggest that thus far no true eukaryotic glucokinases have been identified, neither from plants, nor from fungi or mammalian cells.