The major transcription initiation site of the p27Kip1 gene is conserved in human and mouse and produces a long 5'-UTR

In this report, using a method that identifies capped ends of mRNA molecules together with RNase protection assays, we demonstrate that p27Kip1 transcription is initiated predominantly from a single site which is conserved in the human and mouse genes. Initiation at this site produces a 5'-untranslated region of 472 nucleotides in the human p27Kip1 mRNA and 502 nucleotides in the mouse p27Kip1 mRNA. In addition, several minor transcription start sites were identified for both the mouse and human genes.These results demonstrate that the major transcription initiation sites in the mouse and human p27Kip1 genes are conserved and that the 5'-UTR of the p27Kip1 mRNA is much longer than generally believed. It will be important to consider these findings when designing experiments to identify elements that are involved in regulating the cellular levels of p27Kip1.The cyclin-dependent kinase inhibitor p27Kip1 plays an important role in regulating cell cycle progression by inhibiting the cyclin-Cdk complexes responsible for promoting the transition from G1-phase to S-phase [1,2]. In normal cells, p27Kip1 levels are high in GO and decrease rapidly in late G1 allowing entry into S-phase [3,4]. p27Kip1 has also been implicated as playing a role in tumor formation and progression. p27Kip1 knockout mice display internal organ hyperplasia and are highly prone to pituitary adenomas [5-7]. In human cancers, such as breast [8], prostate [9], colorectal [10], and others, low levels of p27Kip1 expression correlate with decreased survival rates. Since mutations of the p27Kip1 gene are rare in cancer cells [11], loss of p27Kip1 must involve changes in the mechanisms that control its expression.Changes in the levels of cellular p27Kip1 are regulated by multiple mechanisms. In general, transcriptional control of the p27Kip1 gene is believed to play a minor role in regulating p27Kip1 expression. However, recent reports indicate that transcription of the p27Kip1 gene can be activated by neur