THE PHOTOSYNTHETIC ENDOSYMBIONT IN CRYPTOMONAD CELLS PRODUCES BOTH CHLOROPLAST AND CYTOPLASMIC-TYPE RIBOSOMES

Cryptomonad algae contain a photosynthetic, eukaryotic endosymbiont. T he endosymbiont is much reduced but retains a small nucleus. DNA from this endosymbiont nucleus encodes rRNAs, and it is presumed that these rRNAs are incorporated into ribosomes. Surrounding the endosymbiont n ucleus is a small volume of cytoplasm proposed to be the vestigial cyt oplasm of the endosymbiont. If this compartment is indeed the endosymb iont's cytoplasm, it would be expected to contain ribosomes with compo nents encoded by the endosymbiont nucleus. In this paper, we used in s itu hybridization to localize rRNAs encoded by the endosymbiont nucleu s of the cryptomonad alga, Cryptomonas Phi. Transcripts of the endosym biont rRNA gene were observed within the endosymbiont nucleus, and in the compartment thought to represent the endosymbiont's cytoplasm, The se results indicate that the endosymbiont produces its own set of cyto plasmic translation machinery. We also localized transcripts of the ho st nucleus rRNA gene. These transcripts were found in the nucleolus of the host nucleus, and throughout the host cytoplasm, but never in the endosymbiont compartment. Our rRNA localizations indicate that the cr yptomonad cell produces two different of sets of cytoplasmic-type ribo somes in two separate subcellular compartments. The results suggest th at there is no exchange of rRNAs between these compartments. We also u sed the probe specific for the endosymbiont rRNA gene to identify chro mosomes from the endosymbiont nucleus in pulsed held gel electrophores is. Like other cryptomonads, the endosymbiont nucleus of Cryptomonas P hi contains three small chromosomes.