Purification and some properties of a specific nuclease which cleaves transfer RNA precursors from the posterior silk gland of Bombyx mori

A specific endonuclease involved in the processing of tRNA precursors was isolated and partially purified from the posterior silk gland of Bombyx mori, and designated as RNase P.Bmo. This enzyme was shown to catalyze the conversion of 4.5 S precursor RNA to 4.1 S RNA by trimming the 5'-additional segment from the precursor RNA. RNase P.Bmo required divalent cations, Mg2+ or Mn2+. In the presence of these divalent cations, K+ or NH4+ activated the RNase P.Bmo reaction. Optimum pH was observed around 8.0. Ribosomal RNA's and mature tRNA from the silk gland were not cleaved by RNase P.Bmo. A 4.5 S precursor RNA fraction containing formycin, an adenosine analog, was less susceptible to RNase P.Bmo than the normal one. These results indicate that RNase P.Bmo has a high substrate specificity. An additional nuclease(s) was isolated. This activity was assumed to remove the extra 3'-segment of the 4.5 S precursor RNA.

The effect of formycin on the processing of transfer RNA precursors in the posterior silk gland of Bombyx mori

Formycin, an adenosine analog, was used in studies of RNA synthesis by silk glands of silkworms in organ culture. Formycin has been shown to inhibit preferentially the incorporation of [3H]uridine into low molecular weight RNA in the cytoplasm. When silk glands were pulse-chase labeled with [3H]uridine in the presence of formycin, the synthesis of 4.5S tRNA precursors was not affected, as determined by gel electrophoresis. However, the accumulation of mature 4S tRNA was greatly reduced in the formycin-treated system, although the precursors synthesized disappeared from the 4.5S region on chase at the same rate as in the control system. [14C]Formycin was incorporated into the 4.5S precursors but not into 4S tRNA. These findings suggest that the inhibition of tRNA synthesis by formycin is due to the existence of some degradation mechanisms of formycin-containing 4.5S precursor RNA's and the failure to process them into normal 4S tRNA.

The biosynthesis of transfer RNA in insects. II. Isolation of transfer RNA precursors from the posterior silk gland of Bombyx mori

The occurrence of precursors to tRNA in the post-polysomal fraction of the posterior silk gland of Bombyx mori was demonstrated by pulse-chase labeling and DNA-RNA hybridization competition experiments. These precursors had molecular sizes ranging from 4S to 5S on polyacrylamide gel electrophoresis. Analysis of the incorporation of the methyl group from [methyl-14C]methionine revealed that a radioactive peak on polyacrylamide gel appeared in the 4.5S region during brief labeling. This suggested that some methylation occurred at the 4.5S precursor step.