AtSTP3, a green leaf-specific, low affinity monosaccharide-H+ symporter ofArabidopsis thaliana

This paper describes the expression analyses of the AtSTP3 gene of Arabidop sis thaliana, the functional characterization of the encoded protein as a n ew monosaccharide transporter, and introduces the AtSTP gene family. The ki netic properties of the AtSTP3 protein (for sugar transport protein 3) were studied in a hexose transport deficient mutant of Schizosaccharomyces pomb e. AtSTP3 represents a new monosaccharide transporter that is composed of 5 14 amino acids and has a calculated molecular mass of 55.9 kDa. Kinetic ana lyses in yeast showed that AtSTP3 is a low affinity, energy-dependent H+ sy mporter with a K-m for D-glucose of 2 mM. RNase protection analyses reveale d that AtSTP3 is expressed in leaves and floral tissue of Arabidopsis. This expression pattern of the AtSTP3 gene was confirmed in AtSTP3 promoter-bet a-glucuronidase (GUS) plants showing AtSTP3-driven GUS activity in green le aves, such as cotelydons, rosette and stalk leaves and sepals. Wounding cau sed an induction of GUS activity in the transgenic plants and an increase o f AtSTP3 mRNA levels in Arabidopsis wild-type plants. Polymerase chain reac tion analyses with degenerate primers identified additional new AtSTP genes and revealed that AtSTP3 is the member of a large family of at least 14 ho mologous genes coding for putative monosaccharide-H+ symporters (AtSTPs).