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S Ochoa

Publications and source records attributed to S Ochoa.

At least 109 records · Page 6Linked to original sources
PubMed1969

Polypeptide chain initiation in E. coli: isolation of homogeneous initiation factor E2 and its relation to ribosomal proteins.

Previous work has shown that F(2), one of several ribosomal factors involved in polypeptide chain initiation, functions in the binding of formylmethionyl-transfer RNA (fMet approximately tRNA(f)) to a messenger RNA-ribosome complex. F(2) was isolated from 1.0 M ammonium chloride washes of E. coli Q13 ribosomes as a protein homogeneous on polyacrylamide gel electrophoresis at both pH 4.5 and 7.8. Its molecular weight is approximately 80,000. Comparison of electrophoretic patterns of ribosomal proteins from NH(4)Cl-washed and unwashed ribosomes and F(2), at pH 4.5, shows that F(2) corresponds to the slowest-moving component of the proteins derived from unwashed ribosomes. This component is missing from the NH(4)Cl-washed ribosomes. The activity of F(2) is stimulated by two additional factors, initiation factor F(1) and a factor(s) present in a narrow ammonium sulfate fraction of the ribosomal NH(4)Cl wash. The nature of the latter is unknown.

Ammonium Chloride↗
PubMed1969

Polypeptide chain initiation in E. coli: sulfhydryl groups and the function of initiation factor F2.

The activity of the chain initiation factor F(2) in promoting the messenger-dependent binding of formylmethionyl-transfer RNA (fMet approximately tRNA(f)) to purified E. coli ribosomes is inhibited by sulfhydryl-binding reagents, such as N-ethylmaleimide or p-hydroxymercuribenzoate, but prior incubation with guanosine triphosphate (GTP) or with ribosomes largely prevents this inhibition. The effect of GTP suggests that it forms a complex with F(2) whereby "active" sulfhydryl groups become sheltered. Experiments on the time course of the binding reaction, with and without preincubation of F(2) with GTP, and gel filtration experiments with (3)H-labeled GTP lend support to this suggestion.

Benzoates↗
PubMed1969

Polypeptide chain initiation in E. coli: studies on the function of initiation factor F1.

The requirement of initiation factors F(1) (highly purified) and F(2) (electrophoretically homogeneous) for ribosomal binding of N-formylmethionyl transfer RNA (fMet approximately tRNA) at low Mg(2+) concentration (3.5 mM), with the trinucleoside diphosphate ApUpG as messenger, was studied under various experimental conditions with 30S + 50S ribosomes and with 30S subunits alone. The results were qualitatively the same in both cases but the amount of binding was two to three times higher when both 30S and 50S subunits were present. Although there was a virtually absolute requirement for F(2) in all cases, considerable binding occurred at 0 degrees in the absence of added F(1). F(1) addition stimulated binding up to twofold under these conditions. However, at 25 degrees , the temperature at which the reaction is usually carried out, there was very little binding with F(2) alone and addition of F(1) stimulated the reaction five- to sixfold. Contrary to current belief, the GTP analog 5'-guanylyldiphosphonate (GMP-PCP) cannot replace GTP in the binding reaction. In particular, there was but little stimulation of binding (about 1.5-fold) by addition of F(1) to F(2)-containing samples when GMP-PCP was used. In marked contrast, binding was stimulated up to sevenfold by addition of F(1) when GTP was substituted for the analog. Under these conditions, there was an ApUpG and F(1)-dependent hydrolysis of GTP. This is observable with 30S subunits alone and can hardly be related to the occurrence of translocation. The results may be interpreted to mean that a complex relatively stable at 0 degrees , but less stable at 25 degrees , is formed upon addition of F(2) alone. Conversion of the less stable to the more stable form of complex is made possible by addition of F(1). This is accompanied or mediated by cleavage of GTP.

Bacterial Proteins↗
PubMed1967

Replication of viral RNA: RNA synthetase from Escherichia coli infected with phage MS2 or Qb.

An RNA synthetase is formed in Escherichia coli after infection with bacteriophages containing RNA. Specific annealing techniques revealed that, from the very outset of the reaction in vitro, the partially purified enzyme-template complex synthesizes parental-type plus strands, namely, MS2-RNA when isolated from cells infected with MS2 phage and Q(beta)-RNA in the case of cells infected with Q(beta) phage. This is in contrast to the situation found with Q(beta) replicase primed with Q(beta)-RNA, where the initial product is the complementary strand.

Carbon Isotopes↗