Group I introns are found in organellar genomes, in the genomes of eubacteria and phages, and in nuclear-encoded rRNAs. The origin and distribution of nuclear-encoded rRNA group I introns are not understood. To elucidate their evolutionary relationships, we analyzed diverse nuclear-encoded small-subunit rRNA group I introns icluding nine sequences from the green-algal order Zygnematales (Charophyceae). Phylogenetic analyses of group I introns and rRNA coding regions suggest that lateral transfers have occurred in the evolutionary history of group I introns and that, after transfer, some of these elements may form stable components of the host-cell nuclear genomes. The Zygnematales introns, which share a common insertion site (position 1506 relative to the Escherchia cofi small-subunit rRNA), form one subfamily o group I introns that has, after its origin, been inherited through common ancestry. Since the first Zygnematales appear in the middle Devonian within the fossil record, the "1506" group I intron presumably has been a stable component of the Zygnematales small-subunit rRNA coding region for 350-400 million years.Group I introns are characterized by conserved RNA secondary structures essential for splicing and are often capable of self-splicing or require protein factors for excision (1-3). The origin and distribution of group I introns are not understood. Group I introns have been found most often in the organellar and nuclear genomes ofgreen algae, higher plants, and fungi and in the genomes of some eubacteria and phages (3). Since the phage group I introns are readily mobile and the phage genome represents a mosaic of gene segments, it is not possible to address group I intron origin with these sequences (4). Of the organellar group I introns, some contain an open reading frame (ORF) which encodes a sequence-specific endonuclease to mediate their lateral transfer into homologous sequences [intron homing (5)]. Group I intron mobility is also postulated to result from reverse splicing (6).Some group I introns which lack endonuclease coding regions appear to be nonmobile and provide a potentially valuable tool for tracing the evolutionary history of these sequences (2): the presence of a nonmobile group I intron positioned in thq homologous site of the tRNALeu of cyanobacteria and in plastids of photosynthetic lineages that diverged as representatives of the eukaryotic crown group (7, 8) radiation (e.g., green algae, land plants, heterokonts, glaucocystophytes) suggests that this intron was present in the progenitor(s) of these plastids and therefore is at least one billion years old (9). Within eukaryotes, the apparent absence of group I introns within the earliest-diverging amitochondrial and aplastidial Archezoa (see ref. 10 for definition) suggests that they were introduced into the nucleus of later-diverging species (i.e., Metakaryota) by gene transfer from the intron-containing cyanobacterium that gave rise to the plastid [i.e., tRNA'-eu group I intron (4, 9)] or the a purple eubacterium that...

Glycoproteins secreted by Tetrahymena into the culture medium were isolated and the N-glycosidic oligosaccharides analyzed using lectin blots and fluorophore-assisted carbohydrate gel electrophoresis (FACE). Lectin blots showed that the glycoproteins secreted by Tetrahymena contain only N-glycosidic structures of the high mannose type. Further analysis using the FACE technology revealed the presence of four different N-glycosidic structures differing only in the number of mannose residues attached to the core chitobiose unit.

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