ISOLATION OF A FAMILY OF CDNA CLONES FROM RICINUS-COMMUNIS L WITH CLOSE HOMOLOGY TO THE HEXOSE CARRIERS

The sugar transport of the so-called group m transport proteins, among them the two so far identified hexose carries from plants, have some strongly conserved sequences in common. Oligonucleotides coding for tw o conserved regions were constructed to amplify, by polymerase-chain-r eaction (PCR), those mRNA species from Ricinus cotyledons that probabl y code for carrier proteins. Eight different sequences for putative he xose carrier proteins were identified; two of them were completed to f ull-length clones. Transformation into Saccharomyces cerevisiae showed that one of them definitely codes for an active glucose transport sys tem. The other clone was, though transcribed, not expressed in yeast. The expression frequency was determined by Northern analysis and quant itative PCR. All 8 putative hexose carrier clones were present in only very small amounts in poly(A)-RNA of Ricinus tissues. The identified glucose carrier sequence (Hex3) was predominantely expressed in roots and sink leaves, whereas the other functionally so far unidentified fu ll-length clone (Hex1) was mostly expressed in roots, hypocotyl and so urce leaf. Both full-length clones showed little or hardly any express ion in cotyledons.