A third way to answer the question is the analysis of the protein biosynthesis in isolated chloroplasts or in in vitro systems assembled from chloroplast components. The finding that the oligomerous proteins of the photosynthesis membrane consist of subunits that are partially encoded by chloroplasts, partially by the nucleus is an interesting aspect. These complexes are good specimen for studying the interactions between both genomes. The question, how such a co-operation has evolved remains unanswered. Accordingly can no synthesis through the membrane exist.
Transport has to occur after synthesis has been completed.
The additional part is termed transit sequence . It seems to recognize specific receptors at the outside of the chloroplast and specific carriers have to exist that bind to the proteins and transport them through the membrane. In addition is an endoprotease required that transfers the transport form of the protein into its active state. Such an enzyme was isolated from chloroplasts of Chlamydomonas reinhardii . Chloroplasts that have been treated with a protease do not import proteins any more, i.e. the required receptors for the transfer sequences and the carriers have ceased to be in force and without them no protein can be transported.