Maturation and nucleo-cytoplasmic transport of rat-liver ribosomal RNA upon D-galactosamine inhibition of transcription

The hepatocyte protein synthesis defect induced by galactosamine involves hypomethylation of ribosomal RNA

D-galactosamine produces an early defect in protein synthesis, independent of its effects on RNA synthesis. Here we show that the defect in protein synthesis is inherent in purified ribosomal subunits in vitro. Further, galactosamine treatment is associated with an 85% decrease in methylation of ribosomal RNA, involving all sites (2'-0-ribose and base positions), intact ribosomes from galactosamine-treated animals can be methylated to a greater extent than control ribosomes and in vitro methylation restores their functional capacity. Statistical analyses of these data, along with those with a number of other hepatotoxins, reveal a correlation coefficient of r = 0.95 (p less than 0.003) between protein synthetic capacity and ribosomal RNA methylation, and linear regression accounts for more than 90% of the observed variation. In contrast, no relationship was found between nucleolar RNA methylation and protein synthetic capacity. A relationship of borderline statistical significance was found between messenger RNA methylation and protein synthetic capacity, but it does not appear consistent with results obtained after CCl4 intoxication. These results lend strong support to the notion that methylation status of ribosomal RNA is an important control for protein synthesis in quiescent hepatocytes and that net hypomethylation is a common response to such divergent hepatotoxins as CCl4, ethionine and D-galactosamine.

Immunolocalization of nucleolar proteins after D-galactosamine-induced inhibition of transcription in rat hepatocytes. Maintenance of association of RNA polymerase I with inactivated nucleolar chromatin

The fate of defined nucleolar constituents during D-galactosamine-induced inhibition of transcription and the accompanying extensive structural changes such as nucleolar segregation, fragmentation and disappearance of the granular components was studied by light and electron microscopic immunolocalization, using antibodies to different nucleolar components. In contrast to other inhibitors such as actinomycin D, we show that preribosomal components as monitored by a ribosomal protein leave the nucleolus, while a large proportion of RNA polymerase I remains associated with the nucleolar chromatin, i.e. probably the pre-rRNA genes, during inactivation of transcription. These small structures containing the RNA polymerase I are characterized by low electron density and resemble the 'fibrillar centers' of normal nucleoli. The results are discussed in relation to current concepts of the functional topology of the nucleolus.

Changes in nonhistone chromatin proteins from liver of young and adult chickens following D-galactosamine administration

The chromatin proteins soluble in 0.35 M NaCl altered during chicken liver development. Administration of D-galactosamine causes variations in the gel patterns of chromatin proteins from chicken liver. The amount of nonhistone protein with a mol. wt of 150,000 decreased both during liver growth and galactosamine application. The majority of age-specific chromatin proteins were, however, unaffected by galactosamine.

Unusual processing of nucleolar RNA synthesized during a heat shock in CHO cells

Maturation of pre-rRNA has been investigated through heat shock experiments in which pre-rRNA synthesis is successively turned off and turned on. After one hour at 43 degrees C high molecular weight RNA is no longer synthesized and both the methylation and the maturation of pre-rRNA synthesized before heat shock are blocked. After two hours recovery at 37 degrees C, methylation and simultaneous maturation of pre-existing RNA occur while pre-rRNA synthesis is reinitiated only after 7 hours at 37 degrees C. During the first 30 min. at 43 degrees C, a residual synthesis of high molecular weight RNA is observed in the nucleolus with an average molecular weight slightly higher than pre-rRNA (4.6 10(6)). During the recovery period at 37 degrees C, RNA synthesized at 43 degrees C is slowly processed into unusual species (39S, 35S, 29S). No new ribosomal RNA appeared in the cytoplasm. This unusual maturation pathway could be a minor pathway of nucleolar RNA processing in exponentially growing cells.