Cyanobacterial GR6 glutamate-1-semialdehyde aminotransferase: a novel enzyme-based selectable marker for plant transformation

The mutant glutamate-1-semialdehyde aminotransferase (GSA-AT) enzyme encode d by the hemL gene of the gabaculine-resistant cyanobacterium Synechococcus PCC6301 strain GR6 was expressed in tobacco following Agrobacterium-mediat ed transformation of leaf discs. When targeted to plastids, the GR6 hemL ge ne product conveyed gabaculine resistance to transgenic plants. Selection u sing 50 and 100 muM gabaculine was shown to produce two distinct explant ph enotypes: 'greens' and 'whites'. T-1 progeny displayed Mendelian segregatio n ratios, and PCR analysis demonstrated the 'green' phenotype corresponded with the presence of the GR6 hemL gene. Furthermore, 'whites' could be resc ued after 9 days growth on solid media containing between 5 muM and 25 muM gabaculine, allowing the potential use of this system for the isolation of gabaculine-sensitive transformants in mutagenesis screening. The use of GR6 hemL as a selectable marker gene provides a novel enzyme-based method for the selection of transgenic regenerants without the need for antibiotic-res istance markers.