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Regulation of Endogenous Murine Leukemia Virus-related Nuclear and Cytoplasmic RNA Complexity in C57BL/6J Mice of Increasing Age¹

Department of Cell Biology, Mayo Clinic/Foundation, Rochester, Minnesota 55901 [D. L. F., M. J. G.], and National Institute on Aging, Gerontology Research Center [T. O., R. G. C.], Baltimore City Hospitals, Baltimore, Maryland 21224

The nucleotide sequence complexity of murine leukemia virus (MuLV)-related RNA has been measured by RNA-complementary DNA hybridization analysis in nuclear and cytoplasmic RNA isolated from liver and brain of low-leukemia-strain C57BL/6J mice of different ages. In these two tissues, an approximate 1.5- to 2-fold increase in the complexity of steady-state nuclear MuLV-related RNA sequences was observed as a function of age. Maximum complexity was observed with nuclear RNA extracts from old mice and corresponded to roughly 70 to 75% of the total MuLV genome. In contrast to the age-related increase in complexity of nuclear MuLV-related sequences, a consistent 30 to 40% of the total MuLV genome was detected in liver and brain steady-state cytoplasmic RNA, irrespective of animal age. These data suggest that control mechanisms regulating the transcription and/or stabilization of nuclear RNA transcripts of endogenous mouse MuLV-related genomes become less stringent with animal age even in lowtumor mouse strains. The data also support the existence of independent posttranscriptional mechanisms which prevent accumulation of these MuLV-related transcripts in steady-state cytoplasmic RNA and which do not seem to be as subject to the relaxation of stringency as a function of age.

¹ Supported by the Mayo Foundation and by the National Institute on Aging.

² Ph.D. candidate in Pathology, Mayo Graduate School of Medicine, University of Minnesota. To whom requests for reprints should be addressed.

³ Present address: Department of Radiation Biophysics, Faculty of Medicine, University of Tokyo, Tokyo 113, Japan.

Received 10/10/79. Accepted 11/14/79.

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