Structure of Xenopus laevis ribosomal protein L32 and its expression during development

Sequencing and characterization of the Xenopus laevis ribosomal protein L34 cDNA

In this paper, a cDNA homologous to the mammalian ribosomal protein (r-protein) L34 was isolated from a Xenopus laevis oocytes library and named XL34. It encodes a protein of 116 residues with an Mr of 13.2 kDa and a highly basic sequence. The nucleotide (nt) and deduced amino acid (aa) sequence have been compared with the L34 sequence from other species. This analysis showed that the L34 is a protein greatly conserved from prokaryotes to eukaryotes.XL34 mRNA is abundantly present in the whole cytoplasm of oocytes at stages 1 and 2.

Synthesis of ribosomal proteins in developing Dictyostelium discoideum cells is controlled by the methylation of proteins S24 and S31

Ribosomal protein mRNAs left over from growth are selectively excluded from polyribosomes in the first half of Dictyostelium discoideum development. This is due to the fact that they are sequestered by a class of free 40S ribosomal subunits, characterized by possessing a methylated S24 protein. At the time of formation of tight cell aggregates, the methylated S24 is substituted by an unmethylated S24, while protein S31 of the same or other 40S subunits becomes methylated. This leads to a rapid degradation of the ribosomal protein mRNAs.

Regulation of the DNA-binding and transcriptional activities of Xenopus laevis NFI-X by a novel C-terminal domain

The nuclear factor I (NFI) family consists of sequence-specific DNA-binding proteins that activate both transcription and adenovirus DNA replication. We have characterized three new members of the NFI family that belong to the Xenopus laevis NFI-X subtype and differ in their C-termini. We show that these polypeptides can activate transcription in HeLa and Drosophila Schneider line 2 cells, using an activation domain that is subdivided into adjacent variable and subtype-specific domains each having independent activation properties in chimeric proteins. Together, these two domains constitute the full NFI-X transactivation potential. In addition, we find that the X. laevis NFI-X proteins are capable of activating adenovirus DNA replication through their conserved N-terminal DNA-binding domains. Surprisingly, their in vitro DNA-binding activities are specifically inhibited by a novel repressor domain contained within the C-terminal part, while the dimerization and replication functions per se are not affected. However, inhibition of DNA-binding activity in vitro is relieved within the cell, as transcriptional activation occurs irrespective of the presence of the repressor domain. Moreover, the region comprising the repressor domain participates in transactivation. Mechanisms that may allow the relief of DNA-binding inhibition in vivo and trigger transcriptional activation are discussed.

Differential accumulation of mRNA and interspersed RNA during Xenopus oogenesis and embryogenesis

Xenopus oocyte cytoplasmic poly(A)+ RNA has been shown to include two major complex classes: mRNA and interspersed RNA. The former is defined by its translatability, while the latter consists of non-translatable repeat-containing transcripts with unknown functions. In this study we compared the accumulation patterns of total mRNA and a subfamily of interspersed RNA, the XR family (McGrew & Richter, 1989, Dev. Biol. 134, 267-70). The results showed that the XR interspersed RNA level continued to increase throughout oogenesis, while the total mRNA level reached a peak at late stage II and then decreased as much as 40% between stage II and stage VI of oogenesis. In addition we have found that, like mRNA, only about half of the non-translatable XR interspersed RNA underwent deadenylation at oocyte maturation. This result suggested that about half of the interspersed RNA, like certain mRNAs, also contains the U-rich element to protect it from the automatic deadenylation, implying the poly(A) tail of interspersed RNA may play a role during early development.

Copper-dependent metabolism of Cu,Zn-superoxide dismutase in human K562 cells. Lack of specific transcriptional activation and accumulation of a partially inactivated enzyme

The regulation of Cu,Zn-superoxide dismutase by copper was investigated in human K562 cells. Copper ions caused a dose- and time-dependent increase, up to 3-fold, of the steady-state level of Cu,Zu-superoxide dismutase mRNA. A comparable increase was also observed for actin and ribosomal protein L32 mRNAs, but not for metallothionein mRNA which was augmented more than 50-fold and showed a different induction pattern. The copper-induced mRNAs were actively translated as judged from their enhanced loading on polysomes, the concomitantly increased cellular protein levels and an augmented incorporation of [3H]lysine into acid-precipitable material. Cu,Zn-superoxide dismutase protein followed this general trend, as demonstrated by dose- and time-dependent increases in immunoreactive and enzymically active protein. However, a specific accumulation of Cu,Zn-superoxide dismutase was noticed in cells grown in the presence of copper, that was not detectable for other proteins. Purification of the enzyme demonstrated that Cu,Zn-superoxide dismutase was present as a reconstitutable, copper-deficient protein with high specific activity (kcat./Cu = 0.89 x 10(9) M-1.s-1) in untreated K562 cells and as a fully metallated protein with low specific activity (kcat./Cu = 0.54 x 10(9) M-1.s-1) in copper-treated cells. Pulse-chase experiments using [3H]lysine indicated that turnover rates of Cu,Zn-superoxide dismutase in K562 cells were not affected by growth in copper-enriched medium, whereas turnover of total protein was significantly enhanced as a function of metal supplementation. From these results we conclude that: (i) unlike in yeast [Carrì, Galiazzo, Ciriolo and Rotilio (1991) FEBS Lett. 278, 263-266] Cu,Zn-superoxide dismutase is not specifically regulated by copper at the transcriptional level in human K562 cells, suggesting that this type of regulation has not been conserved during the evolution of higher eukaryotes; (ii) copper ions cause an inactivation of the enzyme in intact K562 cells; and (iii) the metabolic stability of Cu,Zn-superoxide dismutase results in its relative accumulation under conditions that lead to increased protein turnover.

Aspects of regulation of ribosomal protein synthesis in Xenopus laevis. Review

The work carried out in the authors' laboratories on the structure and expression of ribosomal protein genes in Xenopus is reviewed, with some comparisons with other systems. These genes form a class that shares several structural features, especially in the region surrounding the 5' ends. These similar structures appear to be involved in coregulated expression that is attained at various regulatory levels: transcriptional, transcript processing and stability, and translational. Particular attention is paid here to the one operating at the translational level, which has been studied during Xenopus oogenesis and embryogenesis, and also during nutritional changes of Xenopus cultured cells. This regulation, which responds to the cellular need for new ribosomes, operates by changing the fraction of rp-mRNA engaged on polysomes, leaving each translated rp-mRNA molecule always fully loaded with ribosomes. Responsible for this translational behaviour is the typical 5'UTR, which characterizes all rp-mRNAs analyzed up to now, and that can bind in vitro some proteins, putative trans-acting factors for this translational regulation.

Determinants of translational fidelity and efficiency in vertebrate mRNAs

This article reviews current knowledge on the mechanisms affecting the fidelity of initiation codon selection, and discusses the effects of structural features in the 5′-non-coding region on the efficiency of translation of messenger RNA molecules.

Primary structure of Xenopus laevis S10, a ribosomal protein that cross-reacts with antibodies to immunoglobulin light chains

Screening of a cDNA expression library from Xenopus laevis splenocytes with purified antibodies to Xenopus immunoglobulin light chains unexpectedly led to the isolation of a clone with an insert whose deduced amino acid sequence is similar to that of a segment of a protein, S10, from the small (40S) subunit of rat ribosomes. A clone containing an insert encoding the corresponding complete protein was isolated from another cDNA library by nucleic acid hybridization. The deduced amino acid sequence of this insert is 94% identical to that of rat S10; no similarity to immunoglobulin sequences could be discerned. The reactivity of the anti-light chain antibodies with the putative Xenopus S10 facilitated the purification of the protein, by high-pressure liquid chromatography, from the 40S subunit of Xenopus ribosomes. Amino-terminal sequence analysis established the identity of the ribosomal protein with the protein encoded by the cDNA insert. To explore the basis for this unexpected cross-reaction, an "antibody transfer" experiment was carried out. Antibodies to Xenopus light chains were adsorbed to Xenopus S10 on a nitrocellulose strip, which was incubated with another strip containing separated heavy and light chains from Xenopus IgM. Antibodies migrated from the strip carrying S10 to the light chains, but not the heavy chains, on the second strip. These results suggest that this unexpected cross-reaction is due to the sharing of one or more epitopes by Xenopus immunoglobulin light chains and the ribosomal protein, S10.

Functional elements of the ribosomal protein L7a (rpL7a) gene promoter region and their conservation between mammals and birds

The transcriptional initiation sites of the chicken ribosomal protein L7a (rpL7a) gene have been determined and found to occur at three consecutive cytidine residues at the start of a polypyrimidine tract of 8 base pairs (bp). A comparative analysis of the 5' upstream regions of the mouse, human and chicken rpL7a genes identified two sequence elements (Box A and Box B) conserved over the 600 million years of divergent evolution that separate mammals and birds. Only Box A (nts - 56 to - 39) and Box B (nts - 25 to - 4) sequences were detected to bind nuclear factors from mouse nuclear extracts in an analysis of the mouse rpL7a 5' upstream sequence. Box A and Box B bind different nuclear factors and the factor binding to mouse Box A and mouse Box B sequences could be effectively competed by corresponding homologous sequences from the human and chicken rpL7a promoters. These results indicate that elements of the rpL7a promoter region are conserved between mammals and birds. An in vivo analysis of the mouse rpL7a 5' upstream sequence required for efficient transcription identified the 5' border of the minimal promoter region as lying between nts - 50 and - 56. Constructs containing 56 bp of 5' upstream DNA and the first 25 bp rpL7a exon were very efficiently transcribed indicating that sequences within the first intron are not required for gene expression. No sequence similarity was detected between the rpL7a promoter elements and described promoter elements of other eukaryotic ribosomal protein genes.

Individual variability in the translational regulation of ribosomal protein synthesis in Xenopus laevis

Ribosomal protein synthesis is regulated by controlling the fraction of mRNA associated with polysomes. It is known that this value changes in different developmental stages during Xenopus embryogenesis or, more generally, with changing cell growth conditions. We present here an analysis of the proportion of mRNA loaded on polysomes, carried out with probes for five different ribosomal proteins on several batches of Xenopus embryos obtained from different individuals. The results obtained indicate the existence of probe-dependent and individual differences, which reflect genetic variations in the cis- and trans-acting regulatory elements responsible for translational regulation. The fraction of ribosomal protein mRNA loaded onto polysomes can be used as an index of an individual's capacity for ribosome production.

Analysis of mRNAs under translational control during Xenopus embryogenesis: isolation of new ribosomal protein clones

We have analyzed several randomly selected mRNAs, of the relatively abundant category, on the basis of maternal or zygotic origin and translational efficiency at different developmental stages. For this purpose, clones from a Xenopus embryo cDNA library were hybridized with cDNA probes prepared with poly(A)+RNA from polysomes and from mRNPs of embryos at different stages. The results obtained indicate that the majority of the relatively abundant mRNAs (38 out of 61) is subject to some kind of translational regulation during embryogenesis. Moreover, 30 clones have been selected as corresponding to mRNAs that behave, from the point of view of transcriptional and translational regulation, similarly to previously studied ribosomal protein (r-protein) mRNAs. Sequence analysis of 20 of these selected cDNAs has shown that half of them are in fact homologous to already sequenced r-protein mRNAs. Unexpectedly we have found that also the mRNA for alpha-cardiac actin and another mRNA homologous to creatine kinase M mRNA have a similar translational regulation during embryogenesis.

Screening a yeast promoter library leads to the isolation of the RP29/L32 and SNR17B/RPL37A divergent promoters and the discovery of a gene encoding ribosomal protein L37

Two promoters (A7 and A23), isolated at random from the Saccharomyces cerevisiae genome by virtue of their capacity to activate transcription, are identical to known intergenic bidirectional promoters. Sequence analysis of the genomic DNA adjacent to the A7 promoter identified a split gene encoding ribosomal (r) protein L37, which is homologous to the tRNA-binding r-proteins, L35a (from human and rat) and L32 (from frogs).