Figure 1

The steps required for a gene to be transferred from an organelle to the nucleus. (a) The gene must be transferred from the organelle, either as a fragment of organellar DNA or as a cDNA, and (b) integrated into a nuclear chromosome. (c) The gene must then acquire the signals for expression, including promoter, terminator, and polyadenylation signals, and also a signal to target the protein back to the organelle. These events may occur together or separately. (d) The expressed gene may be translated on free polysomes to produce a protein that is targeted to mitochondria, or alternatively the mRNA may be targeted to mitochondria to be translated on the surface. (e) The targeting signal must be removed and (f) the protein has to be assembled in order for it to function. Assembly may require re-sorting to the correct location within the organelle and additional processing of sorting signals.