Accumulation of mature mRNA in the nuclear fraction of mammalian cells

Little is known about the nuclear mRNA content of mammalian cells. In this study, me analyzed by Northern blotting with a panel of probes the nuclear and cytoplasmic fractions derived from several rodent cell lines. For most of the genes under study, mature mRNAs could easily be detected in the nucl ear fraction and accumulated to higher levels than the corresponding precur sors. In addition, significant differences in the nucleocytoplasmic partiti on of mature mRNAs were observed between genes as weil as between cell type s (NIH 3T3, CTLL-2, D3-ES, PC-12), indicating that this nuclear accumulatio n of mRNA is regulated, Thus, while it is usually considered that splicing is the limiting step of pre-mRNA processing, these results point towards tr ansport or nuclear retention of mRNA as a key determinant of nuclear mRNA m etabolism (C) 1999 Federation of European Biochemical Societies.