A 5' beta-globin matrix-attachment region and the polyoma enhancer together confer position-independent transcription

Insertions of reporter constructs into the genome of higher eukaryotes typically lead to variegated gene expression due to position effects at the sites of integration. The 20-kb human beta-globin (beta Glb) locus control region (LCR) has been found to dampen these position effects when included in an expression vector. Several studies have indicated that much of the activity of the beta Glb-LCR resides in hypersensitive site II, which contains a strong enhancer. In this study, we have focused on the matrix-attachment region (MAR) at the 5' boundary of the beta Glb-LCR. We find that the beta Glb-MAR, by itself, has little effect on transcription of a reporter gene in stable transformants. However, when the beta Glb-MAR is linked in cis with the polyoma virus enhancer, the MAR-enhancer construct confers high levels of copy-dependent transcription that is independent of the chromosomal site of integration. These results suggest that the beta Glb-MAR may work synergistically with particular enhancer elements to dampen chromosomal position effects and ensure high-level expression.