Restriction fragments containing the 16S rRNA gene of the western aster yellows mycoplasmalike organism (SAY-MLO) were identified in Southern blots probed with cloned fragments of the western X-disease mycoplasmalike organism 16S rRNA gene. Two fragments which contained the entire SAY-MLO 16S rRNA gene and flanking DNA were cloned in M13 and sequenced. The SAY-MLO 16S rRNA gene is approximately 1,535 bp long, has a G+C content of 47 mol%, and has an overall secondary structure similar to that proposed for . Putative rRNA promoter sequences and sequences involved in processing of the primary rRNA transcript were similar in the SAY-MLO, two species, and , suggesting that these prokaryotes and the mycoplasmalike organisms may have similar transcriptional and processing enzymes. We identified two tRNA genes, a tRNA (GTA) gene upstream from the 16S rRNA gene and a tRNA (GAT) gene in the spacer region between the 16S and 23S rRNA genes. Comparisons of the SAY-MLO 16S rRNA nucleotide sequence with 16S rRNA sequences of other organisms indicated that the SAY-MLO is phylogenetically related most closely to other plant-pathogenic mycoplasmalike organisms, followed by species, species, and some species.


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