Dissecting aneurysms of the posterior cerebral artery: angioarchitecture and a review of the literature

Isolated dissecting aneurysms of the posterior cerebral artery are not as rare as previously reported. Affecting primarily a younger population, this condition can be recognized by angiographic patterns, the most common being a "pearl and string" morphology. We reviewed the literature and present a series of six cases, discussing management strategies.

RNA editing in the replication cycle of human hepatitis delta virus

For some time it has been known that the RNA genome of human hepatitis delta virus (HDV) undergoes a specific RNA editing event. This review describes the editing phenomenon and its potential biological significance, and evaluates the data regarding the mechanism involved, including the possible relationship to other RNA editing phenomena.

Editing on the genomic RNA of human hepatitis delta virus

It has been shown previously that during replication of the genome of human hepatitis delta virus (HDV), a specific nucleotide change occurs to eliminate the termination codon for the small delta antigen (G. Luo, M. Chao, S.-Y. Hsieh, C. Sureau, K. Nishikura, and J. Taylor, J. Virol. 64:1021-1027, 1990). This change creates an extension in the length of the open reading frame for the delta antigen from 195 to 214 amino acids. These two proteins, the small and large delta antigens, have important and distinct roles in the life cycle of HDV. To further investigate the mechanism of this specific nucleotide alteration, we developed a sensitive assay involving the polymerase chain reaction to monitor changes on HDV RNA sequences as they occurred in transfected cells. We found that the substrate for the sequence change was the viral genomic RNA rather than the antigenomic RNA. This sequence change occurred independently of genome replication or the presence of the delta antigen. Less than full-length genomic RNA could act as a substrate, but only if it also contained a corresponding RNA sequences from the other side of the rodlike structure, which is characteristic of HDV. We were also able to reproduce the HDV base change in vitro, by addition of purified viral RNA to nuclear extracts of cells from a variety of species.

Sequence-specific recognition of RNA hairpins by bacteriophage antiterminators requires a conserved arginine-rich motif

We have dissected the protein and nucleic acid determinants that direct a group of transcriptional antiterminators to their specific target operons. These antiterminators, the N gene products of phages lambda, 21, and P22, function solely with their respective recognition sites, nut, to modify RNA polymerase to a termination-resistant form. We demonstrate that a unique hairpin sequence within each nut site, called boxB, confers genome specificity by interacting with a small amino-terminal domain of the cognate N protein. This interaction is dependent upon an arginine-rich subdomain, which is conserved not only among the N proteins but also in many RNA binding proteins from ribosomes and RNA virus capsids. Notably, this motif constitutes an essential domain of the HIV protein Tat whose function as a trans-activator requires a specific hairpin sequence.

An antitermination protein engages the elongating transcription apparatus at a promoter-proximal recognition site

As a transcriptional activator, the N protein of phage lambda acts to suppress transcription termination by recognizing a promoter-proximal site, nut, which is separated from the terminators by thousands of base pairs. We demonstrate here that N interacts with the elongating RNA polymerase in transit through the boxB domain of nut. This interaction leads to the stable association of N as an integral component of the transcription apparatus. During subsequent elongation, N translocates along with polymerase through several defined terminators positioned beyond nut. Therefore, by being an operon-specific subunit of the transcription apparatus, N presumably prevents the interaction of polymerase with termination signals.