REGULATION OF PRE-MESSENGER-RNA PROCESSING BY SRC

Background: Changes in gene expression in response to external signals provide a key mechanism for the regulation of higher eukaryotic cell functions. The importance of transcriptional control in the response o f cells to growth factors and cytokines has been extensively documente d, but gene expression has also been shown to be controlled at other l evels, such as the stability of mRNA in the cytoplasm, its localizatio n and translation. By contrast to transcriptional control, little is k nown of the contribution of pre-mRNA nuclear processing to the regulat ion of gene expression, as most of our knowledge of pre-mRNA processin g in vivo is indirect, being inferred from comparisons of transcriptio n rates and levels of mRNA accumulation. Results: In this study, we ha ve used as a model the well-characterized maturation pathway of transc ripts of the cytokine, tumour necrosis factor beta (TNF beta). We have used the murine TNF beta gene as a reporter for pre-mRNA processing, using a co-transfection approach to investigate whether overproduction of proteins involved in signal transduction influences the processing of TNF beta transcripts. Although transfection of both activated ras and src gents led to an increase in RNA accumulation in the nuclear an d cytoplasmic compartments, as expected from their transactivation of the TNF beta expression vector, only src induced a modification of RNA processing. Comparison of several modes of src activation indicated t hat two distinct effects of src on pre-mRNA processing can be uncouple d: one involves slowing down splicing and the other allows the export of partially spliced transcripts. These effects can be observed not on ly on the three introns of TNF beta but also on transcripts from a bet a globin expression vector. Discussion: We have characterized how the processing of transcripts of TNF beta and beta globin is regulated by the signal transduction pathway that includes the Src protein, establi shing that external signals have the capacity to regulate gene express ion at a post-transcriptional level within the nucleus. Src seems to a ct on a general mechanism of splicing and/or mRNA transport, but its b iologically relevant targets are likely to be restricted to genes for which either alternative processing pathways are in competition, or th e kinetics of splicing is critical. This regulation could reflect a mo dulation by Src of the activity of components of the splicing and tran sport machineries, but could also involve RNA-binding proteins, which have been shown to interact with Src.