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J. Biol. Chem., Vol. 278, Issue 45, 44439-44447, November 7, 2003

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A Novel Type of Chloroplast Stromal Hexokinase Is the Major Glucose-phosphorylating Enzyme in the Moss Physcomitrella patens^*

Tina Olsson, Mattias Thelander, and Hans Ronne

From the Department of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, Box 7080, SE-750 07 Uppsala, Sweden

Hexokinase catalyzes the first step in the metabolism of glucose but has also been proposed to be involved in sugar sensing and signaling both in yeast and in plants. We have cloned a hexokinase gene, PpHXK1, in the moss Physcomitrella patens where gene function can be studied directly by gene targeting. PpHxk1 is a novel type of chloroplast stromal hexokinase that differs from previously studied membrane-bound plant hexokinases. Enzyme assays on a knock-out mutant revealed that PpHxk1 is the major glucose-phosphorylating enzyme in Physcomitrella, accounting for 80% of the total activity in protonemal tissue. The mutant is deficient in the response to glucose, which in wild type moss induces the formation of caulonemal filaments that protrude from the edge of the colony. Growth on glucose in the dark is strongly reduced in the mutant. Sequence data suggest that most plants including Physcomitrella and Arabidopsis have both chloroplast-imported hexokinases similar to PpHxk1 and traditional membrane-bound hexokinases. We propose that the two types of plant hexokinases have distinct physiological roles.

Received for publication, June 13, 2003 , and in revised form, August 4, 2003.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY260967 and AY260968.

^* This work was supported by grants from the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning (FORMAS) and the Swedish Research Council (VR). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Dept. of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, Box 7080, SE-750 07 Uppsala, Sweden. Tel.: 46-18-673313; Fax: 46-18-673279; E-mail: Hans.Ronne{at}vbsg.slu.se.

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