A tobacco matrix attachment region reduces the loss of transgene expression in the progeny of transgenic tobacco plants

The RB7 matrix attachment region (MAR), when flanking a uidA (GUS) reporter gene, has been previously shown to increase uidA gene expression by 60-fol d in stably transformed tobacco suspension cell lines. We have now used the same co transformation procedure to determine the effect of flanking MARs on uidA gene expression in tobacco plants. The neomycin phosphotransferase selection gene and uidA reporter gene on separate plasmids were co-transfor med into seedlings by microprojectile bombardment. In primary transgenic pl ants, the average uidA expression in plants with MARs was twofold greater t han in control plants without MARs, but there was no effect on variation of expression. GUS activity was not proportional to the number of integrated uidA transgenes over the entire range of copy numbers. However, in the lowe r part of the copy number range, MAR lines show a tendency for expression t o increase with copy number. Transgene expression in backcross progenies of the MAR-containing lines averaged threefold higher than in control progeni es. MARs also reduced the loss of transgene expression in the BC1 generatio n. Sixty-three per cent of the 21 MAR-containing primary transformants, but only 20% of the 14 control primary transformants, produced backcross proge nies in which no loss of transgene expression was observed. These observati ons are discussed in the context of homology-dependent gene silencing.