THE MAR-MEDIATED REDUCTION IN POSITION EFFECT CAN BE UNCOUPLED FROM COPY NUMBER-DEPENDENT EXPRESSION IN TRANSGENIC PLANTS

To study the role of matrix-associated regions (MARs) in establishing independent chromatin domains in plants, two transgenes were cloned be tween chicken lysozyme A elements, These transgenes were the neomycin phosphotransferase (NPTII) gene under control of the nopaline synthase (nos) promoter and the beta-glucuronidase (GUS) gene controlled by th e double cauliflower mosaic virus (dCaMV) 35S promoter, The A elements are supposed to establish an artificial chromatin domain upon integra tion into the plant DNA, resulting in an independent unit of transcrip tional regulation, Such a domain is thought to be characterized by a c orrelated and position-independent, hence copy number-dependent, expre ssion of the genes within the domain. the presence of MARs resulted in a higher relative transformation efficiency, demonstrating MAR influe nce on NPTII gene expression. However, variation in NPTII gene express ion was not significantly reduced. The selection bias for NPTII gene e xpression during transformation could not fully account for the lack o f reduction in variation of NPTII gene expression, Topological interac tions between the promoter and A element may interfere with the A elem ent as a domain boundary. In contrast, the GUS gene on the same putati ve chromatin domain showed a highly significant reduction in variation of gene expression, as expected from previous results, Surprisingly, no copy number-dependent GUS gene expression was observed: all plants showed approximately the same GUS activity, We concluded, therefore, t hat dCaMV 35S-GUS gene expression in mature tobacco plants is regulate d by some form of dosage compensation.