Transcription of a pre-mRNA in eukaryotic cells elongates from the 5′ to the 3′ end, but intron removal during a pre-mRNA splicing does not always proceed in this orientation. In this study, we identified eight mouse p53 transcripts that retained one or more of introns 6, 7 and 8. The 5′ part of intron 9 was also retained while the 3′ part was not studied. These intron-containing transcripts, abbreviated as p53-ICTs, were detected at low abundance in many mouse embryonic fibroblasts (MEF) as well as cancer cell lines and tissues, and the highest ratio of these p53-ICTs to the mature p53 mRNA was seen in the normal pancreas. Serum starvation decreased those p53-ICTs that retained introns 6 and 7 but increased the levels of those lacking these introns while the level of the mature p53 mRNA was unaffected. Treatment of several cancer cell lines with cisplatin increased the mature p53 mRNA level but decreased these p53-ICTs. Transfection of p53-/- MEF with the p53 cDNA or several p53-ICT mini-genes slightly increased the cell viability and rendered the cells resistant to cisplatin. These data also suggest that p53 pre-mRNA splicing may have multiple orders of intron removal, some of which may not follow the "first come, first served" principle. It remains possible that these p53-ICTs are splicing intermediates existing as a mechanism for the cell to respond more promptly to a demand for more p53 and that p53 protein may be required for a normal life of MEF.
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Acknowledgments This work was supported by a grant from NCI, NIH RO1CA100864 to D. J. Liao. We thank Dr. Fred Bogott at Austin Medical Center, Austin of Minnesota, for his excellent English editing of the manuscript.
- Alternative splicing