Rapid detection of ultraviolet-induced reversion of an amber mutation in mouse L cells

Self major histocompatibility complex class I antigens expressed solely in lymphoid cells do not induce tolerance in the CD4+ T cell compartment

Transgenic mice expressing self major histocompatibility complex (MHC) class I (H-2Kb) antigen solely in lymphoid cell lineages do not acquire tolerance to H-2Kb expressed on skin grafts. H-2Kb-specific cytotoxic T cell responses were completely abrogated in these mice, even after they had rejected skin grafts. Moreover, thymocytes expressing T cell receptors that confer H-2Kb reactivity on cytotoxic CD8+ T cells were eliminated. The ability to reject grafts correlated with the presence of a novel population of H-2Kb-reactive CD4+ T cells. At least some of these CD4+ T cells recognize peptides derived from H-2Kb by processing. We conclude that self MHC I antigens induce tolerance in the CD8 T cell compartment via negative selection when expressed exclusively by lymphoid cells. In contrast, tolerance to MHC class II-restricted self peptides derived by processing of such MHC I antigens is not induced in the CD4 T cell compartment. This suggests that effective transfer of self antigens from lymphoid cells to MHC II-positive cells that can process and present them as self peptides to thymocytes or CD4+ T cells does not take place in vivo. Thus, sequestration of self antigens and MHC II molecules in distinct cell types in the thymic microenvironment allows potentially autoreactive and functionally competent CD4+ T cells that recognize cryptic MHC II-restricted self peptides to mature into the peripheral T cell repertoire under normal physiological circumstances.

Expression of dicistronic transcriptional units in transgenic tobacco

We investigated whether the two cistrons of a dicistronic mRNA can be translated in plants to yield both gene products. The coding sequences of various reporter genes were combined in dicistronic units, and their expression was analyzed in stably transformed tobacco plants at the RNA and protein levels. The presence of an upstream cistron resulted in all cases in a drastically reduced expression of the downstream cistron. The translational efficiency of the gene located downstream in the dicistronic units was 500- to 1,500-fold lower than that in a monocistronic control; a 500-fold lower value was obtained with a dicistronic unit in which both cistrons were separated by 30 nucleotides, whereas a 1,500-fold lower value was obtained with a dicistronic unit in which the stop codon of the upstream cistron and the start codon of the downstream cistron overlapped. As a strategy to select indirectly for transformants with enhanced levels of expression of a gene which is by itself nonselectable, the gene of interest can be cloned upstream from a selectable marker in a dicistronic configuration. This strategy can be used provided that the amount of dicistronic mRNA is high. If, on the other hand, the expression of the dicistronic unit is too low, selection of the downstream cistron will primarily give clones with rearranged dicistronic units.

Expression of dicistronic transcriptional units in transgenic tobacco

We investigated whether the two cistrons of a dicistronic mRNA can be translated in plants to yield both gene products. The coding sequences of various reporter genes were combined in dicistronic units, and their expression was analyzed in stably transformed tobacco plants at the RNA and protein levels. The presence of an upstream cistron resulted in all cases in a drastically reduced expression of the downstream cistron. The translational efficiency of the gene located downstream in the dicistronic units was 500- to 1,500-fold lower than that in a monocistronic control; a 500-fold lower value was obtained with a dicistronic unit in which both cistrons were separated by 30 nucleotides, whereas a 1,500-fold lower value was obtained with a dicistronic unit in which the stop codon of the upstream cistron and the start codon of the downstream cistron overlapped. As a strategy to select indirectly for transformants with enhanced levels of expression of a gene which is by itself nonselectable, the gene of interest can be cloned upstream from a selectable marker in a dicistronic configuration. This strategy can be used provided that the amount of dicistronic mRNA is high. If, on the other hand, the expression of the dicistronic unit is too low, selection of the downstream cistron will primarily give clones with rearranged dicistronic units.