Post-transcriptional polyadenylation is probably an essential step in selection of Balbiani ring transcripts for a cytoplasmic role

Three-dimensional structure of a specific pre-messenger RNP particle established by electron microscope tomography

Electron microscope tomography has been used to refine the structural analysis of individual RNP particles synthesized on Balbiani ring genes in Chironomus tentans. The spherical particles have a diameter of 500 A and are composed of a thick RNP ribbon bent into an asymmetrical, four-domain, ring-like configuration. The first domain, containing the 5' end of the transcript, and the fourth domain, containing the 3' end, are close to each other.

A BR 1 gene in Chironomus tentans has a composite structure: a large repetitive core block is separated from a short unrelated 3'-terminal domain by a small intron

The large Balbiani ring (BR) genes in the dipteran genus Chironomus have been considered to be homogeneous repetitive structures. Analysis of a genomic DNA segment now reveals that a BR 1 gene in C. tentans is a composite gene, consisting of two different types of sequences. A 15-20 kb core block of tandemly arranged repeat units extends close to the 3' end of the BR 1 gene and ends in repetitive structures partly different from the repeat units in the core block. A 55 bp long intron separates the core block, which probably constitutes a single exon, from a non-related 3'-exon, comprising the final 332 bp of the translated part of the gene. According to hydrophobicity and secondary structure predictions, the 3'-exon encoded peptide is distinctly different from the repetitive core block domain and attains a globular structure. The carboxyl-terminal peptide domain is likely to be a general feature of BR encoded proteins and may have important functions in the excretion and polymerisation of the secretory proteins.

Visualization of the formation and transport of a specific hnRNP particle

The growth and maturation of the transcription products on the Balbiani ring (BR) genes in Chironomus tentans has been characterized by electron microscopy. The BR transcript is packed into a series of well defined ribonucleoprotein structures of increasing complexity: a 10 nm fiber, a 19 nm fiber, a 26 nm fiber, and a 50 nm granule. The basic 10 nm element was revealed in Miller spreads. The in situ structure of the transcription products and RNA compaction estimates suggested that the 10 nm fiber is packed into the 19 nm fiber as a tight coil. The transition of the 19 nm fiber into the 26 nm fiber is accompanied by a major change of the basic 10 nm fold into a noncoiled structure. Finally, the 26 nm fiber makes a one and one-third left-handed turn forming the final product, the BR granule. Upon translocation through the nuclear pore the BR granule becomes rod-shaped, which most likely corresponds to a relaxation of the highest-order structure into a straight 26 nm fiber.

Adenosine selectively inhibits labeling of chromosomal RNA, especially hnRNA, probably by acting at or near the site of chain initiation

The effects of adenosine on labeling of nucleolar preribosomal RNA, chromosomal plus nuclear sap hnRNA, and 4-5S RNA in explanted salivary gland cells of chironomus tentans has been studied. Of chromosomal transcripts it is the labeling of polymerase II-promoted RNA that is interrupted preferentially, but 4-5S RNA is influenced as well. The labeling of hnRNA and 4-5S RNA is diminished by 70-90 percent and 45-60 percent, repectively, while the incorporation into the nucleolar preribosomal RNA remains essentially unchanged. Labeled adenosine is transported efficiently across the plasma membrane and becomes phosphorylated to AMP, ADP, and ATP, of which ATP predominates at noninhibitory concentrations. The rate of the formation of [(3)H]AMP is, however, enhanced in response to the increase in external adenosine doses, whereas that of [(3)H]ATP increases only slowly or remains essentially unaltered. A rise in exogenous [(3)H] adenosine concentration to 200 muM yields a [(3)H]ATP/[(3)H]AMP ratio that is about one order of magnitude lower than that at 20 muM of the nucleoside. In parallel with this, there is a gradual repression of the labeling of chromosomal RNA. A similar treatment with guanosine produces only minor reduction in GTP/GMP quotient and does not influence significantly the labeling of any sizable RNA fraction. Thus the experimental data strongly indicate that the purine ribonucleoside adenosine, but not guanosine, gives rise to a markedly diminished triphosphate/monophosphate quotient simultaneously with a selective suppression of the labeling of chromosomal RNA, especially hnRNA, when applied in overdoses. The sequence of hnRNA events during inhibition by adenosine resembles the effect of the purine nucleoside analogue 5,6-dichloro-1-beta-D- ribofuranosylbenzimidazole, indicating that the site of inhibitory action is at or close to the initiation of transcription.