RESUMEN
The control of mRNA stability plays a fundamental role in the regulation of gene expression in plants and other eukaryotes. This control can be influenced by the basal mRNA decay machinery, sequence-specific decay components, and regulatory factors that respond to various stimuli. Important progress has been made towards the identification of some of these elements over the past several years. This is true particularly with respect to cis-acting sequences that control mRNA stability, the identification of which has been the focus of much of the initial work in the field. Characterization of mRNA fragments associated with post-transcriptional gene silencing and two plant transcripts that give rise to detectable decay intermediates have provided insight into the mRNA decay pathways. These, and other studies, are indicative of similarities, as well as of interesting differences between mRNA decay mechanisms in plants and yeast - the system that has been used for most of the pioneering work. Future studies in this area, particularly when enhanced by emerging genetic and genomic approaches, have tremendous potential to provide additional knowledge that is unique to plants or of broad significance.
RESUMEN
We present a deep ASCA observation of a broad absorption line quasar (BALQSO) PG 0946+301. The source was clearly detected in one of the gas imaging spectrometers, but not in any other detector. If BALQSOs have intrinsic X-ray spectra similar to normal radio-quiet quasars, our observations imply that there is Thomson thick X-ray absorption (NH greater, similar1024 cm-2) toward PG 0946+301. This is the largest column density estimated so far toward a BALQSO. The absorber must be at least partially ionized and may be responsible for attenuation in the optical and UV. If the Thomson optical depth toward BALQSOs is close to 1, as inferred here, then spectroscopy in hard X-rays with large telescopes like XMM would be feasible.