Plastids originated from an endosymbiotic event between
an early eukaryotic host cell and an ancestor of
today's cyanobacteria. During the events by which the
engulfed endosymbiont was transformed into a permanent
organelle, many genes were transferred from
the plastidal genome to the nucleus of the host cell.
Proteins encoded by these genes are synthesised in
the cytosol and subsequently translocated into the
plastid. Therefore they contain an N-terminal cleavable
transit sequence that is necessary for translocation.
The sequence is plastid-specific, thus preventing
mistargeting into other organelles. Receptors embedded
into the outer envelope of the plastid recognise
the transit sequences, and precursor proteins are
translocated into the chloroplast by a proteinaceous
import machinery located in both the outer and inner
envelopes. Inside the stroma the transit sequences
are cleaved off and the proteins are further routed to
their final locations within the plastid.