Functional characterization of a tobacco matrix attachment region-mediated enhancement of transgene expression. 
TM2, a new matrix attachment region (MAR) isolated from tobacco, increases transgene expression in plants.  We have carried out a more detailed analysis of the DNA elements in TM2 with the aim of improving its effect on transcription activation.  Our study of the location effect of individual MARs on the expression of the adjacent 35S:gusA cassette indicated that the TM2 functions in a bidirectional manner, with the 5'-MAR being more efficient in enhancing beta-glucuronidase expression than the 3'-MAR.  The influence of 5'-MAR on different linked mini-promoters in transgenic tobacco cells suggested that the role of TM2 depends on the basic expression of the transgenes.  Deletion analysis of one topo II site and two unwinding sites together with one T-box revealed that all these sites contribute most (93.3%) of the transcription activation mediated from the TM2 sequence.  Additionally, micrococcal nuclease accessibility of the 35S promoter region can be strengthened by linked TM2, suggesting that the TM2 mediates the spreading of nucleosome opening.  Taken together, our results reveal that the TM2 mediates a more open and accessible chromatin DNA structure for promoter-dependent active transcription, which in turn enhances transgene expression.