Synthetic human beta‐globin 5′HS2 constructs function as locus control regions only in multicopy transgene concatamers.

Transgenes linked to the beta‐globin locus control region (LCR) are transcribed in a copy‐dependent manner that is independent of the integration site. It has previously been shown that the LCR 5′HS2 region does not require its NF‐E2 dimer binding site for LCR activity. In this paper we analyse synthetic 5′HS2 core constructs containing point mutations in the other factor binding sites 3′ of the NF‐E2 dimer site. The results show that 5′HS2 core is a partially active LCR that functions in a concatamer of at least two copies but not when present as a single copy in transgenic mice and that no single binding site within 5′HS2 is required for position‐independent expression. In addition, the H‐BP factor is identical to upstream stimulatory factor (USF) and full enhancement levels by 5′HS2 core in MEL cells require a combination of all the factor binding sites. We suggest that 5′HS2 cores in a concatamer interact with each other to establish an area of open chromatin and that this process may be the basis of LCR function.