Fast-forwarding evolution. A gene’s jump from the chloroplast genome into that of the nucleus is made visible here through the development of antibiotic resistance. In the two green shoots, the resistance gene has migrated into the cell nucleus, where it can be correctly read, thus allowing the plant to grow on an antibiotic-containing medium. Max Planck Institute of Molecular Plant Physiology |
Chloroplasts,
the plant cell’s green solar power generators, were once living beings
in their own right. This changed about one billion years ago, when they
were swallowed up but not digested by larger cells. Since then, they
have lost much of their autonomy. As time went on, most of their genetic
information found its way into the cell nucleus; today, chloroplasts
would no longer be able to live outside their host cell. Scientists in
Ralph Bock’s team at the Max Planck Institute of Molecular Plant
Physiology have discovered that chloroplast genes take a direct route to
the cell nucleus, where they can be correctly read in spite of their
architectural differences.
Cyanobacteria
are among the oldest life forms, and appear to be the forerunners of
green chloroplasts in plant cells. They do not possess a true cell
nucleus, but their genetic substance is made up of the same four
building blocks as that of humans, plants and animals. Therefore, the
genes encoded in the chloroplast DNA can also be read in the cell
nucleus; indeed, many genes that were still found in the cell organelles
during early evolution are now located exclusively in the genome of the
nucleus. How they made their way there has previously been unclear. Two
mechanisms appeared likely: either direct transport in the form of DNA
fragments from the chloroplasts to the nucleus or transport in the form
of mRNA, which is then transcribed back into DNA.
The
direct transfer of DNA appears to predominate in the chloroplasts, but
this pathway raises two problems. The first problem lies in the
promoters, the DNA sequences which ensure that genes are recognised as
such. They are located upstream of the genes and recruit proteins that
are required for transcription of the genes. However, promoters from
chloroplasts are not recognised as such by the proteins in the nucleus,
so that the DNA reading machinery should overlook these incoming genes.
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.
