AN ACTIVE SUCROSE CARRIER (SCR1) THAT IS PREDOMINANTLY EXPRESSED IN THE SEEDLING OF RICINUS-COMMUNIS L

During the germination of Ricinus communis seedlings sucrose is releas ed from the endosperm and taken up by the cotyledons for their growth and for export to the rapidly growing hypocotyl and roots. Both the me sophyll cells and the phloem of castor bean possess active sucrose car riers. We isolated one of these carriers by heterologous screening wit h the spinach sucrose carrier cDNA for further investigation of sucros e transport at the cellular level. The deduced peptide sequence of thi s sucrose carrier (Scr1) has 12 putative membrane spanning domains and shares homology with other transporters of the major facilitator supe rfamily (MFS). When expressed in Saccharomyces cerevisiae, Scr1 shows active sucrose transport that can be inhibited by PCMBS or CCCP. Accor ding to the pH-dependence the carrier seems related to SUC2 of Arabido psis and Plantago, which are phloem-specific in these plants. It also shows a relatively high specificity for sucrose, compared with maltose or raffinose. Scr1 is expressed in equal amounts in all organs of a 6 -day-old castor bean seedling and to a lesser extent in green sink and source leaves.