Structural and functional analysis of Escherichia coli ribosomes containing small deletions around position 1760 in the 23S ribosomal RNA.

Three different small deletions were produced at a single Pvu 2 restriction site in E. coli 23S rDNA of plasmid pKK 3535 using exonuclease Bal 31. The deletions were located around position 1760 in 23S rRNA and were characterized by DNA sequencing as well as by direct fingerprinting and S1-mapping of the rRNA. Two of the mutant plasmids, Pvu 2-32 and Pvu 2-33, greatly reduced the growth rate of transformed cells while the third mutant, Pvu 2-14 grew as fast as cells containing the wild-type plasmid pKK 3535. All three mutant 23S rRNAs were incorporated into 50S-like particles and were even found in 70S ribosomes and polysomes in vivo. The conformation of mutant 23S rRNA in 50S subunits was probed with a double-strand specific RNase from cobra venom. These analyses revealed changes in the accessibility of cleavage sites near the deletions around position 1760 and in the area around position 800 in all three mutant rRNAs. We suggest, that an altered conformation of the rRNAs at the site of the deletion is responsible for the slow growth of cells containing mutant plasmids Pvu 2-32 and Pvu 2-33.     

A common maturation pathway for small nucleolar RNAs.

We have shown that precursors of U3, U8 and U14 small nucleolar RNAs (snoRNAs) are not exported to the cytoplasm after injection into Xenopus oocyte nuclei but are selectively retained and matured in the nucleus, where they function in pre-rRNA processing. Our results demonstrate that Box D, a conserved sequence element found in these and most other snoRNAs, plays a key role in their nuclear retention, 5' cap hypermethylation and stability. Retention of U3 and U8 RNAs in the nucleus is saturable and relies on one or more common factors. Hypermethylation of the 5' caps of U3 RNA occurs efficiently in oocyte nuclear extracts lacking nucleoli, suggesting that precursor snoRNAs are matured in the nucleoplasm before they are localized to the nucleolus. Surprisingly, m7G-capped precursors of spliceosomal small nuclear RNAs (snRNAs) such as pre-U1 and U2, can be hypermethylated in nuclei if the RNAs are complexed with Sm proteins. This raises the possibility that a single nuclear hypermethylase activity may act on both nucleolar and spliceosomal snRNPs.     

Some effects of antibiotics on bacterial polyribosomes as studied by gel electrophoresis

Bacterial polyribosomes possess characteristic electrophoretic mobilities in agarose-acrylamide composite gels. In cells whose normal protein synthesis is inhibited by certain antibiotics, the resolution of the gel electrophoresis technique has permitted the detection of specific increases in the mobility of the polyribosomes. Antibiotics producing these changes in polyribosome mobility include inhibitors of the 30 S as well as the 50 S subunit.The in vivo action of streptomycin has been studied in some detail. Streptomycin alters the polyribosomes of sensitive strains, haploid as well as heterodiploid, but does not alter polyribosomes of strains resistant to or dependent upon streptomycin. Streptomycin-altered polyribosomes are stable in vivo for more than one hour and exhibit a considerably prolonged run-off time following rifampicin treatment. They are also significantly more resistant to the in vitro RNase degradation than control ribosomes. The subunit composition ($\text{50}\text{S}\text{30}\text{S}$) of the altered polyribosomes remains unchanged from the control (1:1).Since the electrophoretic mobility of monosomes remains unchanged during the antibiotic treatment, the evidence presented suggests that the alteration of polyribosome mobility involves a stacking of the ribosomes on mRNA.     

A mutation at the universally conserved position 529 in Escherichia coli 16S rRNA creates a functional but highly error prone ribosome.

A base substitution of G to U was constructed at position 529 in Escherichia coli 16S rRNA. The U529 mutant ribosomes were functional and present on polysomes but were highly error prone and caused a progressive loss of cell viability. They displayed elevated levels of readthrough of stop codons and frameshifting, and an increase in thermal sensitivity of beta-galactosidase, suggestive of missense errors. These results demonstrate that the university conserved G529 is involved in tRNA selection at the A site during protein synthesis.     

The cell-binding domains of plasminogen and their function in plasma

Plasminogen binding sites are expressed by a wide variety of cell types and serve to promote fibrinolysis and local proteolysis. In this study, the recognition specificity of cells for plasminogen has been examined, primarily using platelets as models. Analyses with plasminogen fragments implicated residues 79-337 (or 353), comprising the first three kringles of plasminogen, as a primary recognition site for plasminogen binding to both thrombin-stimulated and nonstimulated platelets. Other regions of plasminogen, namely residues 354-439 and 442-790, can also participate in the interaction, and these other regions contribute differentially to the binding of the ligand to stimulated and nonstimulated platelets. Binding to nucleated cells, with U937 cells serving as the prototype, is dependent upon a recognition specificity similar to that of unstimulated platelets. Binding of Glu-plasminogen, the native form of the molecule, to thrombin-stimulated platelets has been shown previously to require platelet fibrin. By comparing the interaction of Glu-plasminogen and its degradation product, Lys-plasminogen, with thrombin-stimulated platelets, it is concluded that the cell surface uniquely enhances the affinity of Glu-, but not Lys-plasminogen, for fibrin. Finally, we have demonstrated that cellular receptors and interactive sites within plasminogen are available in the plasma environment. Thus, the functions ascribed to cellular plasminogen receptors can occur within a physiologic setting.     

Dental anthropology. By Juraj Kallay. Tisak: Izdavački zavod Jugoslavenske akademije,Zagreb. x + 181 pp. 459 figures,tables, bibliography,index. N.p.

Exceptions have been cited which rule against the simple autosomal recessive hypothesis for the transmission of susceptibility to infection with Australia antigen (HBsAg). An attempt is made here to present a genetic model for a complex segregation analysis of a new and unique set of data to test this hypothesis. Regression techniques were used to estimate in four populations, age and sexspecific penetrance levels and the frequency of the gene whose product is hypothesized to be HBsAg. While the genetic hypothesis was not in general supported, observed deviations and their possible causes are discussed.