An unspliced group I intron in 23S rRNA links Chlamydiales, chloroplasts, and mitochondria

Chlamydia was the only genus in the order Chlamydiales until the recent cha racterization of Simkania negevensis Z(T) and Parachlamydia acanthamoebae s trains. The present study of Chlamydiales 23S ribosomal DNA (rDNA) focuses on a naturally occurring group I intron in the I-CpaI target site of 23S rD NA from S. negevensis. The intron, SnLSU.1, belonged to the IB4 structural subgroup and was most closely related to large ribosomal subunit introns th at express single-motif, LAGLIDADG endonucleases in chloroplasts of algae a nd in mitochondria of amoebae. RT-PCR and electrophoresis of in vivo rRNA i ndicated that the intron was not spliced out of the 23S rRNA, The unspliced 658-nt intron is the first group I intron to he found in bacterial rDNA or rRNA, and it may delay the S, negevensis developmental replication cycle b y affecting ribosomal function.