Regulation of P120 mRNA levels during lymphocyte stimulation: Evidence that the P120 gene shares properties with early and late genes
✍ Scribed by Amy Wilson; James W. Freeman
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 986 KB
- Volume
- 60
- Category
- Article
- ISSN
- 0730-2312
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✦ Synopsis
PI20 is a growth-regulated nucleolar protein, the expression of which is required for G1-to S-phase transition in lymphocytes. P I 20 appears to be involved in ribosomal biogenesis presumptively through its putative role as a rRNA methyltransferase. To better understand the role of PI20 in cell cycle progression, we examined the regulation of the P I 20 gene in resting lymphocytes and in mitogen-stimulated lymphocytes as they progress from G,-phase toward S-phase. PI20 mRNA was detected after the immediate early gene c-fos and persisted as the cells approached S-phase. A decrease in P I 20 mRNA coincided with the expression of histone H3 mRNA. The level of P I 20 mRNA increased as cells proceeded through G1-phase, and this increase was attributed to a more than threefold increase in the P I 20 transcription rate and an increase in P I 20 mRNA stability. The PI20 gene is transcribed in resting lymphocytes, although the steady-state level of P I 20 is small or nonexistent. P I 20 mRNA accumulates in resting cells in the presence of the protein synthesis inhibitor cycloheximide. Furthermore, the steady-state level of P I 20 mRNA increases in the presence of cycloheximide after PHA-stimulation; this level does not increase in cells not treated with this protein synthesis inhibitor. The presence of cycloheximide increases both the transcription rate of the PI 20 gene and the stability of P I 20 mRNA. These studies indicate that PI20 expression is cell cycle regulated in a complex manner and that the P120 gene has properties of both early and late genes. This time ordered regulation for P I 20 expression may represent a necessary step for the cell cycle associated increase in ribosomal biogenesis that is required for G1-to S-phase transition.