An Unspliced Group I Intron in 23S rRNA LinksChlamydiales, Chloroplasts, and Mitochondria

ABSTRACT Chlamydia was the only genus in the order Chlamydiales until the recent characterization of Simkania negevensis Z T and Parachlamydia acanthamoebae strains. The present study of Chlamydiales 23S ribosomal DNA (rDNA) focuses on a naturally occurring group I intron in the I- Cpa I target site of 23S rDNA from S. negevensis . The intron, SnLSU · 1, belonged to the IB4 structural subgroup and was most closely related to large ribosomal subunit introns that express single-motif, LAGLIDADG endonucleases in chloroplasts of algae and in mitochondria of amoebae. RT-PCR and electrophoresis of in vivo rRNA indicated that the intron was not spliced out of the 23S rRNA. The unspliced 658-nt intron is the first group I intron to be found in bacterial rDNA or rRNA, and it may delay the S. negevensis developmental replication cycle by affecting ribosomal function.