Ultrastructural localization of RNA in cryptomonads

Summary

In a previous study, DNA was localized in cells of two cryptomonads,Pyrenomonas sp. andCryptomonas ovata, by use of immuno-gold technique. Of particular interest was the ultrastructural localization of DNA in the nucleomorph, supposed to be a vestigial nucleus of a former endosymbiont [Hansmann Pet al. (1986) Eur J Cell Biol 42: 152–160]. In the present paper, distribution of RNA in the same two organisms is reported. RNA was detected by the specific and very sensitive RNase-gold method. RNA could be demonstrated in all of the four plasmatic compartments of cryptomonad cells (cytoplasm, periplastidal compartment, mitochondrion, and plastid), although the amounts differed greatly in the respective compartments. In the nucleus, the condensed chromatin and the nucleolus were preferentially labeled. Intense labeling could also be found over the fibrillogranular region of the nucleomorph. This fact lends strong support to the supposition that the fibrillogranular body represents the structural and functional equivalent of a nucleolus and thus again supports the hypothesis that the nucleomorph represents a vestigial eukaryotic nucleus. InPyrenomonas sp., gold-particle density over the nucleolus and the fibrillogranular body was quantitatively evaluated in order to compare their respective RNA synthesizing activities. Labeling density over the nucleolus was found to be 2.7 times higher and thus, on account of its greater volume, the nucleolus may contain 17 times more RNA than the fibrillogranular body of the nucleomorph.