The mechanism of peptide chain termination

Cosuppression of eukaryotic release factor 1-1 in Arabidopsis affects cell elongation and radial cell division

The role of the eukaryotic release factor 1 (eRF1) in translation termination has previously been established in yeast; however, only limited characterization has been performed on any plant homologs. Here, we demonstrate that cosuppression of eRF1-1 in Arabidopsis (Arabidopsis thaliana) has a profound effect on plant morphology, resulting in what we term the broomhead phenotype. These plants primarily exhibit a reduction in internode elongation causing the formation of a broomhead-like cluster of malformed siliques at the top of the inflorescence stem. Histological analysis of broomhead stems revealed that cells are reduced in height and display ectopic lignification of the phloem cap cells, some phloem sieve cells, and regions of the fascicular cambium, as well as enhanced lignification of the interfascicular fibers. We also show that cell division in the fascicular cambial regions is altered, with the majority of vascular bundles containing cambial cells that are disorganized and possess enlarged nuclei. This is the first attempt at functional characterization of a release factor in vivo in plants and demonstrates the importance of eRF1-1 function in Arabidopsis.

Mechanism and regulation of eukaryotic protein synthesis

This review presents a description of the numerous eukaryotic protein synthesis factors and their apparent sequential utilization in the processes of initiation, elongation, and termination. Additionally, the rare use of reinitiation and internal initiation is discussed, although little is known biochemically about these processes. Subsequently, control of translation is addressed in two different settings. The first is the global control of translation, which is effected by protein phosphorylation. The second is a series of specific mRNAs for which there is a direct and unique regulation of the synthesis of the gene product under study. Other examples of translational control are cited but not discussed, because the general mechanism for the regulation is unknown. Finally, as is often seen in an active area of investigation, there are several observations that cannot be readily accommodated by the general model presented in the first part of the review. Alternate explanations and various lines of experimentation are proposed to resolve these apparent contradictions.

Cloning and expression of a mammalian peptide chain release factor with sequence similarity to tryptophanyl-tRNA synthetases

The termination of protein synthesis is encoded by in-frame nonsense (stop) codons. Most organisms use three nonsense codons: UGA, UAG, and UAA. In contrast to sense codons, which are decoded by specific tRNAs, nonsense codons are decoded by proteins called release factors (RFs). Here we report the cloning of a mammalian RF cDNA by the use of monoclonal antibodies specific for rabbit RF. Functional studies showed that, when expressed in Escherichia coli, the protein encoded by this cDNA has in vitro biochemical characteristics similar to those of previously characterized mammalian RFs. DNA sequencing of this eukaryotic RF cDNA revealed a remarkable sequence similarity to bacterial and mitochondrial tryptophanyl-tRNA synthetases, with the greatest similarity confined to the synthetase active site, and no obvious similarity to bacterial RFs.

The function, structure and regulation of E. coli peptide chain release factors

The termination of protein synthesis in Escherichia coli depends upon the soluble protein factors RF1 or RF2. RF1 catalyzes UAG and UAA dependent termination, while RF2 catalyzes UGA and UAA dependent termination. The proteins have been purified to homogeneity, their respective genes isolated, and their primary structures deduced from the DNA sequences. The sequences reveal considerable conserved homology, presumably reflecting functional similarities and a common ancestral origin. The RFs are encoded as single copy genes on the bacterial chromosome. RF2 exhibits autogenous regulation in an in vitro translation system. The mechanism of autoregulation appears to be an in-frame UGA stop codon that requires a 1+ frameshift for the continued synthesis of the protein. Frameshifting prior to the inframe stop codon occurs at a remarkably high frequency by an unknown mechanism. Future studies will be directed at understanding how RFs interact with the ribosomal components, and further defining the mechanism of RF2 frameshifting.

Arthropod ribosomes. Integrity of ribosomal ribonucleic acids from aphids and water fleas

The 28 S aphid rRNA differs from those of the other insects in two points: (1) it lacks the primary nick; (2) it is larger by 0.2 - 106 daltons. The 28 S rRNA from water fleas also, like that from crayfish, is larger than those from comon insects by 0.2 - 106 daltons. These crustacean 28-S rRNAs were shown to contain the primary nick, which is probably not located in the central point. The 18-S rRNAs from aphids and water fleas had molecular weights significantly larger than 0.7 - 106, the common vaalue for the eukaryotic 18 S rRNA. It was was suggested that the 3'-terminal base sequences of these RNAs are different from the common sequences of these RNAs are different from the common sequence proposed for the 18 S rRNAA of eukaryotes. These exceptional characteristics of the rRNAs from the parthenogenetic animals may provide a probe for general functions of the rRNA in the eukaryotic ribosomes.