The endosymbiotic theory provides an explanation for the origin of chloroplasts. Which trait of chloroplasts supports this explanation?

The endosymbiotic theory explains that mitochondria and chloroplasts descended from the same type of bacteria. According to the endosymbiotic theory, a host cell ingested symbiotic bacteria, which specialized into chloroplasts. The evidence of the theory is that mitochondria and chloroplasts have their own circular DNA transcriptional and translational machinery.