GROUP-I TWINTRONS - GENETIC ELEMENTS IN MYXOMYCETE AND SCHIZOPYRENID AMEBOFLAGELLATE RIBOSOMAL DNAS

Protists are unicellular eukaryotes which represent a significant frac tion of the global biodiversity. The myxomycete Didymium and the schiz opyrenid amoeboflagellate Naegleria are distantly related protists. Ho wever, we have noted several striking similarities in life cycle, cell morphology, and ribosomal DNA organization between these organisms. B oth have multicopy nuclear extrachromosomal ribosomal DNAs. Here the s mall subunit ribosomal RNA genes are interrupted by an optional group I twintron, a novel category among the group I introns. Group I twintr ons are mobile self-splicing introns of 1.3-1.4 kb in size, with a com plex organization at the RNA level. A group I twintron consists of two distinct ribozymes (catalytic RNAs) with different functions in RNA p rocessing, and an open reading frame encoding a functional homing endo nuclease-all with prospects of application as molecular tools in biote chnology. Updated RNA secondary structure models of group I twintrons, as well as an example of in vitro ribozyme activity, are presented. W e suggest that the group I twintrons have been independently establish ed in myxomycetes and schizopyrenid amoeboflagellates by horizontal ge ne transfer due to a combination of the phagocytotic behavior in natur al environments and the extrachromosomal multicopy nature of ribosomal DNA. (C) 1998 Elsevier Science B.V. All rights reserved.