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The Id family of helix-loop (HLH) proteins is thought to affect the balance between cell growth and differentiation by negatively regulating the function of basic HLH transcriptional factors. Although Id is involved in cell cycle regulation, little is known about S phase-dependent regulation. To gain insight on S phase-dependent regulation of Id-1 gene, cell cycle regulation of Id-1 has been investigated in HeLa cells. The level of Id-1 mRNA transiently decreases at 1 hour after release from aphidicolin block and gradually restored since 2 hour. Otherwise H2B histone (S phase-dependent induction) mRNA was reciprocally expressed with Id mRNA. Id mRNA was decreased by actinomycin-D but increased in the cells after addition of cycloheximide. In DNase I footprinting analysis, the nuclear factors interacting with the cis-elements were identified in Non-S and S phase cells: CREB/ATF (-1017), SBE (-993) binding site and Egr-1 (-1063) were detected. The trans-acting factors were bound to each cis-element-specific but quantitative differences were not shown between Non-S and S phase cells. In DNA mobility shift assay using oligonucleotide containing CREB/ATF binding site or SBE site on the Id-1 promoter, one DNA-protein complex was identified in nuclear extract prepared from Non-S and S phase cells, respectively. CREB/ATF was reduced to S phase-dependent manner in HeLa cells but SP1-NF1 and Egr-1 was not changed. The deletion mutants of Id-1 promoter did maintain S phase-independent activity of luciferase during cell cycle. These results suggest that Id-1 mRNA is transiently repressed to S phase-dependent manner in HeLa cells. The biological significance of the S phase-dependent repression of Id-1 mRNA is not known.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]