The second difficulty is in the correct processing of the gene sequence. Genes consist of several modules, separated by non-coding DNA regions (introns). Since the introns obstruct protein synthesis, they need to be removed from the mRNA, a procedure described as splicing. The whole process, ending in synthesis of the correct protein, can resume only once this has taken place. Once again, however, the mRNA is processed differently in the cell nucleus than in the chloroplasts, and for a long time, chloroplast introns seemed to have been an insurmountable hurdle for the correct reading of chloroplast genes in the nucleus.
"But they are actually nothing of the sort", stresses Ralph Bock, head of the research group. "Our trials have shown that the introns are recognised in the cell nucleus and spliced out, even if not always at exactly the same sites as might have been the case in the chloroplasts." Functional proteins are formed despite this. It is thought that the introns even help the splicing enzymes by folding themselves into stable RNA structures, thus directing the enzymes to the right locations. At the same time, the RNA structure seems to help the ribosomes find the correct starting point for protein synthesis.
Since the transfer of genes into the cell nucleus is an extremely slow evolutionary process, which has taken nature millions of years, it has not been possible to investigate the underlying mechanism to date. However, researchers have now managed to fast-forward this gene transfer in the laboratory. Because the cells were subjected to high selection pressure, the transference of genes from the chloroplasts into the nucleus became essential for survival, so that it could be made readily visible. It was found that the transfer takes place without the involvement of RNA and that the DNA apparently jumps directly from the cell's chloroplasts into its nucleus.
|Contact: Professor Dr. Ralph Bock|