PRIMARY AND SECONDARY STRUCTURE ANALYSES OF THE RDNA GROUP-I INTRONS OF THE ZYGNEMATALES (CHAROPHYTA)

The Zygnematales (Charophyta) contain a group-I intron (subgroup IC1) within their nuclear-encoded small subunit ribosomal DNA (SSU rDNA) co ding region. This intron, which is inserted after position 1506 (relat ive to the SSU rDNA of Escherichia coli), is proposed to have been ver tically inherited since the origin of the Zygnematales approximately 3 50-400 million years ago. Primary and secondary structure analyses wer e carried out to model group-I intron evolution in the Zygnematales. S econdary structure analyses support genetic data regarding sequence co nservation within regions known to be functionally important for in vi tro self-splicing of group-I introns. Comparisons of zygnematalean gro up-I intron secondary structures also provided some new insights into sequences that may have important roles in in vivo RNA splicing. Seque nce analyses showed that sequence divergence rates and the nucleotide compositions of introns and coding regions within any one taxon varied widely, suggesting that the ''1506'' group-I introns and rDNA coding regions in the Zygnematales evolve independently.