REPRESSION OF HISTONE GENE-TRANSCRIPTION IN QUIESCENT 3T6 FIBROBLASTS

Maintaining murine 3T6 fibroblasts in serum-depleted medium for a peri od of three days results in a resting cell population that does not sy nthesize DNA. Histone mRNA levels, closely tied to the cell-proliferat ion rate, are low due to a reduced rate of synthesis. A comparison of histone gene transcription in vitro by nuclear extracts of quiescent o r proliferative 3T6 cells showed that a 200-bp segment of the promoter was responsible for repressing gene activity when cells were in a G0 state. In the absence of the distal promoter region (- 200 to - 400), gene transcription remained high in quiescent cells, indicating the pr oximal promoter region (+ 1 to - 200) was responsible for basal gene a ctivity. Alterations in protein binding to the distal promoter region correlated with histone H4 gene activity, suggesting that repression o f histone gene transcription is linked to the attachment of a specific nuclear protein. During G1, the histone H4 gene was efficiently trans cribed in vitro, but an inability to process the histone pre-mRNA limi ted the cellular content of mature histone mRNA. This distinction betw een transcriptional (in G0) and post-transcriptional (in G1) mechanism s for modulating histone mRNA levels suggests that gene-regulatory fac tors are specifically activated in quiescent cells to reduce expressio n of non-essential genes.