Nucleotide sequence from a ribosomal RNA transcription unit of Xenopus laevis

I have determined the sequence of the central part of a ribosomal transcription unit from Xenopus laevis, using the plasmid pXlr101. The sequence comprises over 200 bp at the 3' end of the 18S gene, the first internal transcribed spacer, the 5.8S gene, the second internal transcribed spacer, and over 100 bp at the 5' end of the 28S gene. The two transcribed spacers have G + C contents of over 80% and include long homopolymeric tracts of G or C (10-15 residues). ITS1 also has long tracts of purines containing several A residues. The Xenopus sequence is compared to other organisms where data is available. The gene sequences show a high level of homology with sequences from other eukaryotes and also some homology with the prokaryote E. coli. No sequence homology is found between the internal transcribed spacers of Xenopus and yeast (Saccharomyces). Tentative secondary structure models are proposed for the Xenopus sequence and again compared to possible models from other organisms. Secondary structure may be highly conserved within the mature rRNAs, even in regions where the primary sequence is variable between species. In the transcribed spacers one hairpin may be held in common by Xenopus and yeast but other secondary structures are not obviously conserved. I have attempted to characterise some ribosomal RNA precursors in Xenopus tissue culture cells by both 'Northern' transfers and SI nuclease protection mapping. Various artifacts limit the usefulness of these techniques in this system. However it is proposed that a putative '30S' precursor exists containing the RNA of 5.8S, ITS2 and 28S, and having the same 5' end as 5.8S rRNA. Evidence from the sequence supports the proposition that 5.8S rRNA in eukaryotes is structurally equivalent to the 5' end of 23S rRNA in E. coli The results lead to speculation of the relationship between rRNA processing in eukaryotes and E. coli

Genetic activity along 315 kb of the Drosophila chromosome

Transcripts from different tissues were mapped along a 315 kb segment of the Drosophila chromosome, a region which includes the rosy and Ace loci. Forty-three distinct RNA species were detected, though only 12 recessive lethal complementation groups had been mapped in the interval. The sum of the sizes of the transcripts covers 33% of the genomic DNA. The distribution of transcription units along the walk is very uneven. Sixty-three kb of genomic DNA at the proximal end of the walk encode 18 transcripts, while only seven are found in the next 153 kb. Each tissue exhibits a specific spectrum of transcripts. No clustering was seen among genes expressed coordinately. In salivary glands, the number of transcripts detected corresponds to the number of chromomeric units in the polytene chromosomes of this tissue. Moreover, the density distribution of transcripts along the DNA walk is parallel to the density distribution of chromomeric units

Drosophila melanogaster acetylcholinesterase gene: structure, evolution and mutations

Acetylcholinesterase is a key component of cholinergic neurotransmission. In Drosophila melanogaster, acetylcholinesterase is encoded by the Ace locus. We have determined the complete organization of the locus. The transcription unit is 34 kb (1 kb = 10(3) bases) long and encompasses ten exons. We have mapped the 5' end of the transcript, sequenced all the intron/exon boundaries, as well as the 3' end of the transcript. The deduced mature transcript is 4291 nucleotides long without poly(A). Sequencing of the promoter region reveals a potential TATA box and (GA)n motives. The Drosophila coding sequence is more split than its vertebrate counterparts, but the splicing sites of the two last exons are precisely conserved among Drosophila and vertebrate cholinesterases, and intriguingly also with the bovine thyroglobulin gene. Finally, a number of the mutations isolated in earlier genetic work are precisely placed on our molecular map in introns, exons and promoter regions. Among them, for example, a short deletion known to affect acetylcholinesterase level and tissue distribution removes promoter regions and the first non-coding exon

Evolution of genes and genomes on the Drosophila phylogeny

Comparative analysis of multiple genomes in a phylogenetic framework dramatically improves the precision and sensitivity of evolutionary inference, producing more robust results than single-genome analyses can provide. The genomes of 12 Drosophila species, ten of which are presented here for the first time (sechellia, simulans, yakuba, erecta, ananassae, persimilis, willistoni, mojavensis, virilis and grimshawi), illustrate how rates and patterns of sequence divergence across taxa can illuminate evolutionary processes on a genomic scale. These genome sequences augment the formidable genetic tools that have made Drosophila melanogaster a pre-eminent model for animal genetics, and will further catalyse fundamental research on mechanisms of development, cell biology, genetics, disease, neurobiology, behaviour, physiology and evolution. Despite remarkable similarities among these Drosophila species, we identified many putatively non-neutral changes in protein-coding genes, non-coding RNA genes, and cis-regulatory regions. These may prove to underlie differences in the ecology and behaviour of these diverse species