STRUCTURAL-ANALYSES OF PLASTID-DERIVED 16S RIBOSOMAL-RNAS IN HOLOPARASITIC ANGIOSPERMS

Higher-order structures have been constructed for plastid-encoded smal l-subunit(SSU, 16S), rRNAs from representatives of seven nonphotosynth etic holoparasitic angiosperm families: Apodanthaceae, Cynomoriaceae, Cytinaceae, Balanophoraceae, Hydnoraceae, Mitrastemonaceae, and Raffle siaceae. Whereas most pairwise comparisons among angiosperms differ by 2-3% in substitutions, the 16S rRNAs of the holoparasites show an inc reasingly greater number of mutations: Cynomorium (7.3%), Cytinus (8.0 %), Bdallophyton (12.7%), Mitrastema (14.9%), Hydnora (19.4%), Pilosty les (30.4%) and Corynaea (35.9%). Despite this high level of sequence variation, SSU structures constructed for all species except Pilostyle s possess the typical complement of 50 helices (that contain numerous compensatory mutations) thereby providing indirect evidence supporting their functionality. Pilostyles, likely with the most unusual plastid 16S rRNA yet documented, lacks four major helices and contains length y insertions for four others. Sequences of products generated via RT-P CR show that these structural modifications are present on a mature (t ranscribed) rRNA. The trend toward increasing numbers of base substitu tions in the holoparasites is accompanied by a marked increase in A+U content of the rRNA. This 'A/T drift' phenomenon of rDNA is especially apparent in Corynaea whose SSU rDNA sequence is 72% A+T. A comparison of Cytinus to tobacco showed that substitution rates appear to be dep endent upon the composition of neighboring bases. Transversions repres ented 26% of the mutations when flanking bases were G or C whereas tra nsversions increased to 36% when the flanking bases were A to T. The u nderlying molecular mechanism associated with these high substitution rates is presently unknown, however, relaxation of selection pressure on ribosome function resulting in altered DNA replication and/or repai r systems may be involved.