Cis‐acting sequences and trans‐acting factors required for constitutive expression of a microinjected HSP70 gene after the midblastula transition of Xenopus laevis embryogenesis

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Abstract
Microinjected human HSP70 promoter-chloramphenicol acetyl transferase (CAT) chimeric genes are constitutively expressed immediately after the midblastula transition of Xenopus embryogenesis. Analysis of a series of 5′-deletion mutants in the HSP70 promoter revealed that sequences within 74 bases of the transcriptional start site were sufficient for strong basal activity. We investigated the role of specific sequences in the basal promoter by injecting HSP70-CAT vectors containing linker-scanner mutations in the basal elements (CCAAT, purine-rich element, GC-element, ATF/AP1, and 1ATA). Our data reveal that deletion of any of these cis-acting elements in the basal promoter prevents expression after the midblastula stage of development. Furthermore, we have identified specific binding activities in embryonic nuclear extracts that complex with basal promoter elements (CCAAT, ATF, and GC) of the heterologous HSP70 promoter. These trans-acting factors are detectable in nuclear extracts of early blastula embryos, and their respective binding activity increases dramatically after the midblastula transition. The expression of the human HSP70 gene after the midblastula transition of Xenopus embryogenesis requires an array of cisacting elements, which interact with specific Xenopus transcription factors.