The bacterial endosymbiont Carsonella ruddii has a 160Kb genome; just 182 genes. This bare-bones gene set may indicate that C. ruddii is on its way to becoming an organelle.
FYI, reference:
Nakabachi A et al. The 160-kilobase genome of the bacterial endosymbiont carsonella. Science (2006) 314(5797):267.