MECHANISMS OF MARCKS GENE ACTIVATION DURING XENOPUS DEVELOPMENT

The myristoylated alanine-rich protein kinase C substrate (MARCKS) is a high affinity cellular substrate for protein kinase C, The MARCKS ge ne is under multiple modes of transcriptional control, including cytok ine- and transformation-dependent, cell-specific, and developmental re gulation, This study evaluated the transcriptional control of MARCKS g ene expression during early development of Xenopus laevis, Xenopus MAR CKS was highly conserved with its mammalian and avian homologues; its mRNA and protein were abundant in the maternal pool and increased afte r the mid-blastula transition (MBT). The Xenopus MARCKS gene was simil ar to those of other species, except that a second intron interrupted the 5'-untranslated region. By transiently transfecting XTC-2 cells an d microinjecting Xenopus embryos with reporter gene constructs contain ing serial deletions of 5'-flanking MARCKS sequences, we identified a 124-base pair minimal promoter that was critical for promoter activity , Developmental gel shift assays revealed that a CBF/NF-Y/CP-1-like fa ctor and an Sp1-like factor bound to this region in a manner correlati ng with the onset of Xenopus MARCKS transcription at MBT. Mutations in the promoter that abolished binding of these two factors also complet ely inhibited transcriptional activation of the MARCKS gene at MBT, Th e binding sites for these two factors are highly conserved in the huma n and mouse MARCKS promoters, suggesting that these elements might als o regulate MARCKS transcription in other species. These studies not on ly increase our knowledge of the transcriptional regulation of the MAR CKS genes but also have implications for the mechanisms responsible fo r zygotic activation of the Xenopus genome at MBT.