Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/1999
Publication Date: N/A
Interpretive Summary: Genes in bacteria are generally located on uninterrupted segments of DNA. In contrast, genes in other living creatures, including algae and amoebae, are interrupted by "intron" sequences that have dubious origins and purposes. Typically, these introns are automatically removed when a gene is used by a cell. Group I introns have never been found in the ribosomal genes of bacteria, but we have discovered a "group I" intron in the 23S ribosomal gene of the bacterium Simkania. Simkania is a member of the order Chlamydiales, which are intracellular pathogens of livestock and other animals. Several introns that are distantly related to this group I intron have been found in algae and amoebae, but unlike these the Simkania intron is not removed from the RNA. The retained intron is in the heart of the bacteria's protein synthetic machinery. Current models of ribosomal structure suggest that this intron should prevent Simkania from making proteins. Analysis of this intron will help us understand how these bacteria survive, the workings of the protein synthetic machinery, and the mechanisms of gene transfer to bacteria.
Technical Abstract: Introns have not previously been found in the ribosomal genes of bacteria. We report the discovery of a group I intron located between positions 1930- 1931 in the large 23S ribosomal gene of Simkania Z, a recently identified member of the bacterial order Chlamydiales. The intron, SnLSU#1, was predicted to form a secondary structure consistent with typical group I intron splicing; however, analysis of in vivo RNA and PCR indicated that the intron was not spliced out. The intron also did not autocatalytically splice out in vitro. SnLSU#1 encoded a 143-residue protein, EndA, that was most closely related to homing endonuclease I-CpaI. I-CpaI and its closest homologs facilitate intron insertion into naive genes and are expressed by 23S rDNA introns in chloroplasts and mitochondria of algae and amoebae. Chlamydiae, mitochondria, and chloroplasts live or grow only within eukaryotes, so SnLSU#1 may have been acquired by Simkania through horizontal transfer. The unspliced 658-base intron was situated in a small, highly conserved hairpin in domain IV of the 23S rRNA, close to the peptidyl-transferase center at the interface between the ribosomal subunits. The location of the unspliced intron in the rRNA could affect ribosomal function and may be responsible for delayed maturation in Simkania's developmental cycle of replication.