Tobacco cultures transformed with cyclin-promoter-gus constructs reveal a discrepancy between gus mRNA levels and GUS protein activity upon leaving the stationary state

beta-Glucuronidase (gus) is widely used as a reporter gene to study transcr iptional regulation in transgenic plants, based on the assumption that the level of GUS activity is indicative of the rate of gus transcription driven by the promoter investigated. Here we examined tobacco BY-2 cultures trans formed with Arabidopsis cyclin promoters fused to gus. In cultures leaving the stationary state, levels of the gus mRNA remarkably increased, indicati ng that the plant cyclins are transcribed only in actively dividing cells. Nevertheless, at the same time GUS protein activity decreased. Our observat ions highlight the need for special precautions while using the gus reporte r gene in conditions that represent a rapid change in a developmental or a metabolic state. These precautions are expected to hold true also for other reporter genes encoding relatively stable proteins. (C) 1999 Elsevier Scie nce Ireland Ltd. All rights reserved.