Effect of human placental ribonuclease inhibitor in cell-free ribosomal RNA synthesis

Genetic screen to dissect protein-protein interactions: ribonuclease inhibitor-ribonuclease A as a model system

Protein-protein interactions are critical for the function of biological systems. Here, we describe a means to dissect a protein-protein interaction. Our method is based on the in vivo interaction between a target protein and the peptide epitopes derived from its partner. This interaction is detected by using hybrid proteins in which the target protein and peptide epitopes are fused to the DNA-binding domain of the lambda repressor protein. An interaction prevents the transcription of a reporter gene. The efficacy of this approach is demonstrated with the ribonuclease inhibitor protein and ribonuclease A, which form a complex with an equilibrium dissociation constant in the femtomolar range. Our method can enable the identification of residues important in a designated protein-protein interaction and the development of antagonists for that interaction.

Transcriptional regulation of genes for plant-type ribulose-1,5-bisphosphate carboxylase/oxygenase in the photosynthetic bacterium, Chromatium vinosum

The content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the photosynthetic purple sulfur bacterium, Chromatium vinosum, grown either heterotrophically or autotrophically, was highly correlated with the level of 2.0-kb mRNA encoding genes for both large (rbcL) and small (rbcS) subunits. This result indicates the transcriptional regulation of Rubisco biosynthesis in Chromatium cells. In the analysis of transcripts for rbcL and rbcS in Escherichia coli transformed by a plasmid bearing both genes downstream of E. coli tac promoter (pCKS1), the mRNAs were found to be the same sizes as those from Chromatium. However, we were unable to detect mRNA for Rubisco in E. coli harboring a plasmid containing the genes for Rubisco and its own promoter without any E. coli promoters (pCUB1). In the in vitro transcription experiment of pCKS1 and pCUB1 by E. coli RNA polymerase, it was observed that the enzyme could not recognize the Rubisco promoter. Therefore, we have purified RNA polymerase from Chromatium cells and developed a homologous in vitro transcription system. We have detected factor(s) for transcriptional regulation from either heterotrophically or autotrophically grown cells of Chromatium using the homologous in vitro transcription system.

Isolation and properties of a single-strand 5'----3' exoribonuclease from Ehrlich ascites tumor cell nucleoli

A single-strand-specific, nucleolar exoribonuclease from Ehrlich ascites tumor cells has been isolated and purified free from other nucleases. The exonuclease degraded single-stranded RNA processively from either a 5'-hydroxyl or a 5'-phosphorylated end and released 5'-mononucleotides. The enzyme digested single-strand poly(C), poly(U), and poly(A) equally well but did not degrade duplex poly(C).poly(I) or poly(A).poly(U). Less than 0.2% of duplex DNA or 1.5% of heat-denatured DNA was degraded under the conditions which resulted in greater than 26% degradation of RNA. The ribonuclease required Mg2+ (0.2 mM) for optimum activity and was inhibited by ethylenediaminetetraacetic acid but not by human placental RNase inhibitor. The native enzyme had a Stokes radius of 42 A and a sedimentation coefficient (S20,w) of 4.3 S. From these values, an apparent molecular weight of 76 000 was derived by using the Svedberg equation. The localization and unique mode of degradation suggest a role for the 5'----3' exoribonuclease in ribosomal RNA processing.

The effect of a ribonuclease inhibitor from human placenta on the in vitro synthesis of human placental proteins

Addition of a ribonuclease inhibitor (10 micrograms/ml) from human placenta caused 2-3-fold increase of [3H]leucine incorporation in the wheat germ extract as directed by human placental poly (A)-mRNA. Analysis of the translated products by sodium dodecyl sulfate/polyacrylamide gel electrophoresis/fluorography revealed that the inhibitor preferentially increased the yields of the larger proteins, particularly those of larger than Mr 40 000. In the presence of the inhibitor, yields of two placental proteins (human placental lactogen and human chorionic gonadotropin) were increased about 70-80% as detected by immunoprecipitation with specific homologous antisera. The method provided an improvement of translation system for studying biosynthesis of other human placental proteins.