Retinoic acid response element in HOXA-7 regulatory region affects the rate, not the formation of anterior boundary expression.

Since it is known that a 307 bp fragment of the position specific regulatory element of human HOXA-7 contains two (DR3 and DR5) retinoic acid response elements (RAREs) at its 3' end, we constructed several deletion constructs containing different numbers of RAREs and examined their effects in vitro and in vivo. The 5' deletion constructs, BM112 and OM213, retaining both RAREs, were highly responsible (about 8 fold induction) for RA in F9 teratocarcinoma cells, versus NM307 (4-5 fold). The construct NS218, with both RAREs deleted but retaining the 5' sequences lost RA responsibility completely, whereas NR271, with one RARE(DR5) deleted retained a 50% inducibility (2.5 fold). In vivo transgenic analysis revealed that the constructs NM307 and NR271, but not OM213 nor BM112, directed the position specific expression pattern. Sequence analysis revealed that HOXA-7 enhancer sequences, including the RARE repeat sequences, were well conserved in human, mouse and chick. Part of the RAREs overlaps with the CDX1 binding site, and sequences of the DR3 RAREs were identical in this species. Two GAGA binding sites were also found to be strictly conserved. Because OM213, which had one GAGA site disrupted but retaining both RAREs, did not direct anterior boundary formation in transgenic mice, these results suggest the importance of the 5' 94 bp region, including the GAGA binding site, in anterior boundary formation and the involvement of the RARE in the rate of expression not in anterior boundary formation.

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