Infection and Maintenance of Holospora Species in Paramecium caudatum

Abstract

Infectious forms of Holospora species have a cytoplasmic region and a periplasmic lumen with an electron-translucent invasion tip. Bacterial 89-kDa pro- teins with two actin-binding motifs translocate from the invasion tip lumen to outside the tip when infectious forms are ingested in digestive vacuoles of a Paramecium cell. With the invasion tip, the bacteria disrupt the digestive vacuole membrane, appear in the host cytoplasm, migrate to their target nucleus, distinguish host nuclear envelopes of two kinds through affinity between the bacterial lipopolysaccharides the outer membranes and an unknown substance of the target nuclear envelope, and then penetrate the target nuclear envelope with the invasion tip. The first bacterium invades the host nucleus within 10 min after mixing. After endosymbiosis the bacteria alter host gene expression, providing an excellent opportunity for elucidating host- symbiont interactions between eukaryotic and prokaryotic cells and establishment of new associations leading to eukaryotic cell evolution