Ribosomal RNA evolution by fragmentation of the 23S progenitor: Maturation pathway parallels evolutionary emergence

Summary The eukaryotic 5.8S and the chloroplast 4.5S ribosomal RNAs were proposed to have arisen from the 5 and 3 ends respectively of prokaryotic 23S ribosomal RNA by the introduction of new processing sites during evolution. This hypothesis was supported by comparison of previously published primary sequences; in addition we can draw models of secondary structure in accord with this notion. Finally, we further noted that the sequence of processing cuts in the maturation pathway of ribosomal RNA reflects the probable order in which they arose during evolution.     

Fine structure of ribosomal RNA: I. Conservation of homologous regions within ribosomal RNA of eukaryotes

By molecular hybridization experiments the homologies between ribosomal RNAs from a unicellular organism (Gyrodinium cohnii), three invertebrates (Drosophila hydei, Chironomus thummi, Sciara coprophila), an amphibian (Xenopus laevis), and a mammal (mouse) were determined. Competition hybridization experiments demonstrated that portions of these homologous regions are the same in all the ribosomal RNAs tested, regardless of animal species. This conclusion based on hybridization data was confirmed by comparative fingerprint analysis. The ribosomal RNA sequences involved in heterologous hybridization have a higher A + T composition than the bulk ribosomal RNA. It appears from competition experiments of a heterologous hybridization that two thirds of the conserved similar regions are present in 18 S ribosomal RNA, and the remaining one third in 28 S ribosomal RNA. It is argued that these similar regions have been conserved during evolution due to their structural and/or functional role in ribosomal RNA.     

Further Studies on the Ribosomal RNA Cistrons of SCIARA COPROPHILA (Diptera)

Additional experiments with homologous as well as heterologous hybridization confirmed our previous finding in Sciara coprophila that XX females have nearly twice the number of ribosomal RNA cistrons as XO males. A comparison between two different X' chromosomes revealed that only the one carrying the irradiation-induced Wavy mutation has a deletion of 70% of its ribosomal RNA cistrons as compared to the standard X. The deletion is relatively stable, and the remaining ribosomal RNA cistrons donot appear to undergo disproportionate replication or magnification as in Drosophila. Homologous hybridization experiments revealed an unusually low reiteration of ribosomal RNA cistrons in this fly, 45 gene copies per X chromosome. The question is raised as to whether such a low number of cistrons may be related to the unusual nucleolar condition encountered in the Sciaridae.     

Multiple effects of temperature,photoperiod and food quality on the performance of a pine sawfly

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Isolation and characterization of the ecdysone receptor and its heterodimeric partner ultraspiracle through development in Sciara coprophila

Regulation of DNA replication is critical, and loss of control can lead to DNA amplification. Naturally occurring, developmentally regulated DNA amplification occurs in the DNA puffs of the late larval salivary gland giant polytene chromosomes in the fungus fly, Sciara coprophila. The steroid hormone ecdysone induces DNA amplification in Sciara, and the amplification origin of DNA puff II/9A contains a putative binding site for the ecdysone receptor (EcR). We report here the isolation, cloning, and characterizing of two ecdysone receptor isoforms in Sciara (ScEcR-A and ScEcR-B) and the heterodimeric partner, ultraspiracle (ScUSP). ScEcR-A is the predominant isoform in larval tissues and ScEcR-B in adult tissues, contrary to the pattern in Drosophila. Moreover, ScEcR-A is produced at amplification but is absent just prior. We discuss these results in relation to the model of ecdysone regulation of DNA amplification.     

Fatty Acid-Induced Modulation of Ouabain Responsiveness of Rat Na,K-ATPase Isoforms

Membrane phospholipids represent a potential influence on the enzymatic properties of the Na,K-ATPase. Little is known concerning the effects of the fatty acid environment surrounding the enzyme on the kinetic properties of the Na,K-ATPase. We used the most obvious difference among the α isoforms of rat, their affinities for digitalis glycosides, to examine the relationship between the lipid environment and the Na,K-ATPase. Specific membrane environments that differ in their fatty acid composition were produced by drug-induced diabetes, as well as variations in diet. The α1 isoforms in various tissues were then characterized by their resistance to ouabain in Na,K-ATPase-enriched membrane microsomal fractions. The Na,K-ATPase activity in nerves and hearts were altered by diabetes and partially restored in nerves after a fish oil diet. Evaluation of enzyme kinetics (dose-response curves for ouabain) in membrane preparations allowed us to correlate the ouabain affinity of α1 isoform with fatty acid composition. The affinity of the α1 isoform for ouabain was significantly increased with accretions in the total amount of fatty acids of the n-6 series (P < 0.0001). Our observations provide a partial explanation for the observed difference in isoform properties among tissues. Moreover, these results underline the interaction between membrane fatty acids and the glycoside binding site of the Na,K-ATPase α1 subunit. Received: 15 June 1998/Revised: 18 November 1998     

Xenopus U3 snoRNA docks on pre-rRNA through a novel base-pairing interaction

U3 small nucleolar RNA (snoRNA) is essential for rRNA processing to form 18S ribosomal RNA (rRNA). Previously, it has been shown that nucleolin is needed to load U3 snoRNA on pre-rRNA. However, as documented here, this is not sufficient. We present data that base-pairing between the U3 hinges and the external transcribed spacer (ETS) is critical for functional alignment of U3 on its pre-rRNA substrate. Additionally, the interaction between the U3 hinges and the ETS is proposed to serve as an anchor to hold U3 on the pre-rRNA substrate, while box A at the 5' end of U3 snoRNA swivels from ETS contacts to 18S rRNA contacts. Compensatory base changes revealed base-pairing between the 3' hinge of U3 snoRNA and region E1 of the ETS in Xenopus pre-rRNA; this novel interaction is required for 18S rRNA production. In contrast, base-pairing between the 5' hinge of U3 snoRNA and region E2 of the ETS is auxiliary, unlike the case in yeast where it is required. Thus, higher and lower eukaryotes use different interactions for functional association of U3 with pre-rRNA. The U3 hinge sequence varies between species, but covariation in the ETS retains complementarity. This species-specific U3-pre-rRNA interaction offers a potential target for a new class of antibiotics to prevent ribosome biogenesis in eukaryotic pathogens.     

A DNase I hypersensitive site flanks an origin of DNA replication and amplification in Sciara

In chromosomes of metazoa, the assembly of the genome into chromatin makes an important but poorly understood contribution to determining where DNA replication will initiate. We addressed this issue by studying the developmental progression of the location of the DNA replication origin (ORI) and alterations in chromatin structure in one of the best-mapped ORIs in metazoa, that found in DNA puff II/9A of the fly Sciara coprophila. We found that DNA synthesis for both normal chromosomal endoduplication and DNA amplification initiates within the same 5.5 kb EcoRI fragment. We showed that irrespective of the mode of ORI function--replication or amplification--chromatin over the 1 kb major ORI is never remodeled into a conventional DNase I hypersensitive site (DH site). Instead, we found that the major site of alterations to chromatin structure at this locus is a large (approximately 400 bp) DH site located 600 bp away from the major ORI, at a position where the frequency of replication initiation events falls dramatically. We describe a tight positive correlation between ORI activity, strength of this DH site, and the intranuclear titer of protein factor(s) that bind the DH site in a sequence-specific manner. We propose that the Sciara replicator in locus II/9A is composed of sequences that reside within the ORI per se as well as sequences encompassed by the DH site.     

Industrial microbiology of solar salt production

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