To understand the mutations and genetic rearrangements that allow rabies virus infections of new hosts and adaptation in nature, the quasispecies structure of the nucleoprotein and glycoprotein genes as well as two noncoding sequences of a rabies virus genome were determined. Gene sequences were obtained from the brain and from the salivary glands of the original host, a naturally infected European fox, and after serial passages in mice, dogs, cats and cell culture. A relative genetic stasis of the consensus sequences confirmed previous results about the stability of rabies virus. At the quasispecies level, the mutation frequency varies, in the following order : glycoprotein region (21n9i10 N4 mutations per bp), noncoding sequence nucleoproteinphosphoprotein region (7n2-7n9i10 N4 mutations per bp) and nucleoprotein gene region (2n9-3n7i10 N4 mutations per bp). These frequencies varied according to the number, type of heterologous passages and the genomic region considered. The shape of the quasispecies structure was dramatically modified by passages in mice, in which the mutation frequencies increased by 12-31i10 N4 mutations per bp, depending on the region considered. Nonsynonymous mutations were preponderant particularly in the glycoprotein gene, stressing the importance of positive selection in the maintenance and fixation of substitutions. Two mechanisms of genomic evolution of the rabies virus quasispecies, while adapting to environmental changes, have been identified : a limited accumulation of mutations with no replacement of the original master sequence and a less frequent but rapid selective overgrowth of favoured variants.

Bovine respiratory syncytial virus (BRSV) is a major cause of respiratory disease in young cattle. Here we demonstrate BRSV persistence at low levels in tracheobronchial and mediastinal lymph nodes up to 71 days after the experimental infection of calves. Positive results were obtained on viral genomic RNA and messenger RNA coding for the nucleoprotein, glycoprotein (G), and fusion protein (F). G and F proteins were also detected in the pulmonary lymph nodes by immunohistochemistry. Double-staining experiments revealed that viral antigen was present in B-lymphocytes. Coculture experiments with the lymph node cells showed that the virus was still able to infect permissive target cells, even though no cytopathic effect was recorded. In vitro studies indicate that BRSV was still able to replicate in bovine B-lymphocyte cell lines 6 months after infection. These results may also be relevant to the understanding not only of the epidemiology and the peculiarities of the immune response of BRSV infections but also of human respiratory syncytial virus infections.

In vivo, the steady-state level of c-myc mRNA is mainly controlled by posttranscriptional mechanisms. Using a panel of transgenic mice in which various versions of the human c-myc proto-oncogene were under the control of major histocompatibility complex H-2K b class I regulatory sequences, we have shown that the 5 and the 3 noncoding sequences are dispensable for obtaining a regulated expression of the transgene in adult quiescent tissues, at the start of liver regeneration, and after inhibition of protein synthesis. These results indicated that the coding sequences were sufficient to ensure a regulated c-myc expression. In the present study, we have pursued this analysis with transgenes containing one or the other of the two c-myc coding exons either alone or in association with the c-myc 3 untranslated region. We demonstrate that each of the exons contains determinants which control c-myc mRNA expression. Moreover, we show that in the liver, c-myc exon 2 sequences are able to down-regulate an otherwise stable H-2K mRNA when embedded within it and to induce its transient accumulation after cycloheximide treatment and soon after liver ablation. Finally, the use of transgenes with different coding capacities has allowed us to postulate that the primary mRNA sequence itself and not c-Myc peptides is an important component of c-myc posttranscriptional regulation.In recent years, it has been realized that the rates of RNA decay in the nucleus and in the cytoplasm are important in determining the level of expression of a gene (reviewed in reference 5). In eukaryotic cells, the range of mRNA stability can vary over several orders of magnitude, and numerous studies have indicated that several mRNAs contain within them the information necessary to determine their stability. Transiently expressed genes such as those encoding proto-oncogenes and growth factors have far shorter half-lives than others, such as those encoding ␤-globin or albumin (15,32). The mechanisms of differential degradation are an attractive problem which covers several distinct aspects, such as analysis of the structural features of mRNA that determine its susceptibility to decay, identification of the locations of the destabilizing determinants, determination of the trans-acting factors with which they interact, and discovery of the physiological signals which alter rates of mRNA decay (reviewed in reference 30).

We show here in mouse embryonal carcinoma (EC) cells that the endo A gene is negatively regulated and shares negative transacting factors with the Py and SV40 viruses. The products of the proto-oncogene c-myc derepress at the transcriptional level the appropriately initiated expression of the endo A gene and activate the Py early promoter in EC stem cells. C-myc products also activate the endo A and the Py early promoters in TDM epithelial cells, and the Py early promoter in 3T6 cells in which the two genes are already expressed or can be expressed. Furthermore we show that the myc exon 1 is essential for activation and that this activation might be mediated by AP1 family factors.

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