The effect of MAR elements on variation in spatial and temporal regulationof transgene expression

The level of transgene expression often differs among independent transform ants. This is generally ascribed to different integration sites of the tran sgene into the plant genome in each independently obtained transformant (po sition effect). It has been shown that in tobacco transformants expressing, for example, a cauliflower mosaic virus (CaMV) 35S promoter-driven beta -g lucuronidase (GUS) reporter gene, these position-induced quantitative diffe rences among individual transformants were reduced by the introduction of m atrix-associated regions (MAR elements) on the T-DNA. We have previously sh own by imaging of in planta firefly luciferase (luc) reporter gene activity that quantitative differences in transgene activity can be the result of e ither a variation in (1) level, (2) spatial distribution and/or (3) tempora l regulation of transgene expression between independent transformants. It is not known which of these three different aspects of transgene expression is affected when the transgene is flanked by MAR elements. Here we have us ed the firefly luciferase reporter system to analyse the influence of MAR e lements on the activity of a CaMV 35S-luc transgene in a population of inde pendently transformed tobacco plants. Imaging of in planta LUC activity in these tobacco plant populations showed that the presence of MAR elements do es not result in less variation in the average level of transgene expressio n between individual transformants. This result is different from that obta ined previously with a 35S-GUS reporter gene flanked by MAR elements and re flects the differences in the stability of the LUC and GUS reporter protein s. Also the variation in spatial patterns of in vivo LUC activity is not re duced between independent transformants when the transgene is flanked by MA R elements. However, MAR elements do seem to affect the variation in tempor al regulation of transgene expression between individual transformants. The potential effects of MAR elements on the variability of transgene expressi on and the relation to the stability of the (trans)gene product are discuss ed.