Sequence analysis of28Sribosomal DNA fromtheamphibian Xenopus laevis

We havedetermined thecompletenucleotide sequenceofXenopuslaevis28S rDNA(4110bp). In orderto locateevolutionarily conservedregionswithin rDNA,we comparedtheXenopus28Ssequenceto homologous rDNAsequencesfrom yeast,Physarum,and E. coli. Numerousregionsof sequencehomologyare dispersed throughout theentirelengthofrDNAfromallfourorganisms. These conservedregionshavea higherA+Tbasecomposition thantheremainder of the rDNA. TheXenopus28SrDNAhasninemajorareasof sequenceinserted when comparedto E. coli23SrDNA. Thetotalbasecomposition of theseinsertsin Xenopusis 83%G+C, and is generallyresponsible for the high(66%)G+C contentof Xenopus28SrDNAas a whole.Althoughthelengthof theinserted sequencesvaries,the insertsare foundin the samerelativepositions in yeast26S,Phvsarum 26S,andXenopus28SrDNAs. In one insertthereare 25 basescompletely conservedbetweenthe variouseukaryotes, suggesting that thisareaisimportant foreukaryotic ribosomes. Theotherinsertsdifferin sequencebetweenspecies andmayormaynotplaya functional role.