Biosynthesis of low molecular weight RNA in mouse myeloma cells.

The low molecular weight RNAs in the nucleus and cytoplasm of mouse myeloma cells have been characterized. There are major nuclear species (other than 5-S and tRNA), four of which contain 'capped' 5' termini. There are three major small cytoplasmic RNAs none of which contain 'caps'. The biosynthesis of the nuclear species when studied using [3H]adenosine as an RNA precursor is characterized by a rapid, transient appearance in the cytoplasm, followed by fixation in the nucleus. Within 15 min, the amount in the cytoplasm has reached a steady-state level maintained for 60 min, while the accumulation in nuclei is linear after a short lag (less than 5 min). When biosynthesis is studied using [Me-3H]methionine as a precursor, much less labeled RNA is present in the cytoplasm, suggesting that methylation may immediately precede fixation into the nucleus.