Inhibition of translation of lens mRNAs in a messenger dependent reticulocyte lysate by cap analogues

The effect of aging on the structure and function of liver messenger RNA

The size of the poly(A) segment of poly(A)+RNA extracted from hepatocytes isolated from 4- to 30-month-old rats was determined using polyacrylamide-agarose gel electrophoresis. The average size of the poly(A) segment isolated from newly synthesized poly(A)+RNA was 150 nucleotides, and the average size of the poly(A) segment isolated from the steady state pool of cytoplasmic poly(A)+RNA was 70 nucleotides. No significant age related change in either the size or the heterogeneity of the newly synthesized poly(A) segment was found. The translational activity of poly(A)+RNA isolated from livers of 4- to 30-month-old rats was determined using wheat germ extract and rabbit reticulocyte lysate cell-free systems. No age-related change in the translational activity of the poly(A)+RNA was observed. The effect of aging on the cap structure was also studied by measuring the inhibition of cell-free translation by 7-methyl guanosine-5'-monophosphate. No age-related change in the sensitivity of the poly(A)+RNA preparations to inhibition of translation by 7-methyl guanosine-5'-monophosphate was observed. Therefore, the results of this study demonstrate that no major age-related change occurs in the structure of the poly(A) segment, the translational activity, or the cap structure of poly(A)+RNA from rat liver.

Delta crystallins and their nucleic acids

delta-Crystallin is a major structural protein of avian and reptilian lenses that is absent from the lenses of fish, amphibia and mammals. It appears to be a tetrameric protein with a native molecular weight near 200 000 (200K) and polypeptide molecular weight near 50K and 48K) (see Note added in proof). The alpha-crystallin polypeptides are extremely similar, associate in various combinations of four and are held together by hydrophobic interactions. Although principally cytoplasmic, delta-crystallin may associate with the cell membrane. delta-Crystallin differs from other lens crystallins in its alpha-helical content, native and subunit molecular weights, antigenicity, low wavelength of maximum fluorescence emission (315 nm) after excitation at 280 nm and amino acid composition (high in leucine; low in aromatic residues en no cysteine). Analyses of peptides, native and subunit molecular weights, and circular dichroism spectra indicate that the primary, secondary, tertiary and subunit structures of delta-crystallin have been generally conserved during evolution. There are at least two tandemly arranged delta-crystallin containing 13-15 introns in the chicken; a similar structure exists for a cloned delta-crystallin gene in the duck. Experiments with chicken show that delta-crystallin synthesis occurs principally in the embryo, especially during lens fiber cell differentiation. delta-Crystallin synthesis also takes place during lens fiber cell differentiation in culture. There is evidence for both transcriptional and post-transcriptional regulation of delta-crystallin synthesis. Current studies on the crystallographic and primary structures of delta-crystallin, on the structure, evolution and expression of the delta-crystallin genes, and on the translation of delta-crystallin mRNAs make this specialized lens protein an active area of investigation.

Structural characterization of the proteins encoded by adenovirus early region 2A

Proteins encoded by adenovirus type 2 and type 5 early region 2A isolated from infected HeLa cells were compared to translation products of E2A-specific messenger RNA in a reticulocyte cell-free system and in Xenopus oocytes. The main cell-free translation product is a 72,000 Mr polypeptide which in HeLa cells as well as in Xenopus oocytes is converted into a 75,000 Mr phosphoprotein capable of binding to single-stranded DNA. Some minor proteins are proteolytic cleavage products of the major protein. In the cell-free system, three E2A polypeptides, 32,000, 37,000 and 44,000 Mr, are translated from minor polyadenylated mRNA species that can be separated from the major mRNA. Synthesis of all E2A polypeptides in vitro is inhibited by cap-analogs. The 44,000 Mr protein is also synthesized in Xenopus oocytes. Tryptic peptide maps of [35S]methionine-labeled E2A proteins were constructed using high pressure liquid chromatography and the position of the methionyl residues within each peptide was determined by amino acid sequencing procedures. This information and the DNA sequence of the adenovirus 5 E2A gene published by Kruijer et al. (1981) were used to align the peptides and to construct a map of the E2A proteins. Our data demonstrate that the major 75,000 Mr protein is coded for by a leftward reading frame of 529 amino acid residues located between 62 and 66 map units. The data also map six sites as targets for proteolytic enzymes. The minor E2A translation products have the same carboxy terminus as the major protein. The initiation codons of the 44,000, 37,000 and 32,000 Mr polypeptides probably correspond to amino acids 170, 243 or 244 and 290 of the major protein. Some functional properties of the major E2A protein are shared by the minor proteins and thus could be mapped. Major sites of phosphorylation, the region involved in binding to single-stranded DNA and the antigenic regions recognized by immune sera are located between amino acid residues 50 to 120, 170 to 470 and 170 to 240, respectively.

alpha A-crystallin messenger RNA of the mouse lens: more noncoding than coding sequences

The 14S messenger RNA (1300 to 1500 nucleotides) for the alpha A chain of alpha-crystallin of the mammalian lens is nearly three times larger than required to code for the polypeptide that contains 173 amino acids. As a means of accounting for this anomaly, a complementary DNA clone for the mouse alpha A-crystallin messenger RNA was constructed in pBR322 and sequenced. Derivation of the protein sequence from the nucleic acid sequence showed that mouse alpha A-crystallin is similar to that of other organisms. The messenger RNA contains 536 nucleotides located on the 3' side of the coding region, excluding the polyadenylate stretch. This 3' sequence does not encode any other crystallin and has multiple termination codons in the three possible reading frames.

Preferential translation of mRNAs in an mRNA-dependent reticulocyte lysate

Messenger RNA competition experiments were performed in an mRNA-dependent reticulocyte lysate using three kinds of mRNA: rabbit globin mRNA, calf eye lens mRNA and RNA of turnip yellow mosaic virus. Our results indicate that at supersaturating concentrations of mRNA preferential translation of certain mRNA species can be observed. Furthermore, the pattern of mRNA selection by the translational apparatus suggests that the rate of translation of different mRNA species is limited by different components of the reticulocyte lysate. Our observations n the cell-free system are strikingly different from our previously published mRNA competition experiments in Xenopus oocytes using the same preparations of lens and globin mRNA, in which no selective translation was observed [Asselbergs et al., Eur. J. Biochem. 94, 249-254 (1979)]. The restraints on mRNA translation in vitro are apparently different from those in vivo, i.e. in oocytes.

Messenger RNA competition in living Xenopus oocytes

When calf lens crystallin mRNA and rabbit globin mRNA are competing for factors limiting protein synthesis in living Xenopus oocytes, no mRNA species is preferentially selected for translation. Differences in the intrinsic translational efficiency of the mRNA species exist, but the relative efficiencies are the same at high and low mRNA concentrations. mRNAs already being translated, in particular endogenous oocyte mRNAs, are less sensitive to competitive inhibition by injected mRNAs. As injected mRNAs gradually become incorporated into the protein-synthesizing machinery of the oocyte, they acquire the same status as the oocyte's own active mRNAs. Exogenous mRNAs this become endogenous mRNAs. These results, together with previous estmates of the translational efficiency of injected heterologous mRNA species, are compatible with the assumption that a large proportion of the endogenous mRNAs is not competing for the translational apparatus of the oocyte and, therefore, probably is present in the temporarily inactivated form.

Post-translational assembly of lens alpha-crystallin in the reticulocyte lysate and in Xenopus laevis oocytes

Lens mRNA was translated in reticulocyte lysate predominantly into monomeric alpha-crystallin chains. Lens polyribosomes added to the cell-free system produced the same polypeptides, but these were detected predominantly in alpha-crystallin aggregates. Lens mRNA, after microinjection into Xenopus laevis oocytes, produced alpha-crystallin subunits that were exclusively found in the form of high-molecular-weight complexes. Also after injection of the purified 14-S mRNA, coding for the alphaA subuint, the synthesized alpha-A polypeptides were incorporated into high-molecular-weight aggregates. In contrast, the synthesis of alphaB subunits, directed by a 10-S mRNA, did not result in aggregate formation. The experiments thus suggest that aggregate formation of alpha-crystallin is triggered by its alphaA subunits, which are then joined by the alphaB subunits. This process occurs partly in the cell-free system and completely in Xenopus oocytes.