SARS STIMULATE BUT DO NOT CONFER POSITION-INDEPENDENT GENE-EXPRESSION

Two minimal scaffold-associated regions (SARs) from Drosophila were te sted in stably transformed cells for their effects on the expression o f reporter genes. The expression of genes bounded by two SARs is consi stently stimulated by about 20- to 40-fold, if the average of a peal o f cell transformants is analyzed. However, analysis of individual, sta ble cell transformants demonstrates that flanking SAR elements do not confer position-independent expression on the reporter gene and that t he extent of position-dependent variegation is similarly targe with or without the flanking SAR elements. The SAR stimulation of expression is observed in stable but not in transiently transfected cell lines. T he Drosophila scs and scs' boundary elements, which do not bind to the nuclear matrix in vitro, are only about one-tenth as active as SARs i n stimulating expression in stable transformants. Interestingly, the S AR stimulatory effect can be blocked by a fragment containing CpG isla nds (similar to 70% GC), if positioned between the SAR and the enhance r. In contrast, when inserted in the same position, control fragments, such as the scs/scs' elements, do not interfere with SAR function.