Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2003
Publication Date: 3/1/2004
Citation: Lee, I., Zhao, Y., Bottner, K.D. 2005. Identification of insertion sequence (IS)-like elements in aster yellow group phytoplasmas. Int J Syst Evol Microbiol: 54:337-347. Interpretive Summary: Phytoplasmas are unusual bacteria that lack a cell wall and that cause several hundred economically important diseases in plants worldwide. A phytoplasma causing severe yellows disease in China aster is termed aster yellows (AY) phytoplasma. This and related phytoplasmas form a large and diverse group of bacteria that cause more than 100 diseases in different plant species including ornamental, vegetable, grain, fruit, and forest crops. To understand how the AY group phytoplasmas cause diseases and how this phytoplasma group has become so genetically diverse, we initiated a project to explore a possible explanation. This paper documents the discovery of novel repetitive insertion sequence (IS)-like elements present in AY group phytoplasma strains. IS elements are mobile genetic elements, commonly present in plant and animal pathogenic bacteria, and evidence indicates that they affect chromosomal rearrangements and the expression of genes related to pathogenicity. This is the first report of IS-like elements in the AY phytoplasma group. The findings will be of great interest to research scientists and students of microbiology and plant pathology.
Technical Abstract: Novel insertion sequence (IS)-like elements were isolated and characterized from phytoplasma strains in the aster yellows (AY) group (16SrI). An analysis of a repetitive DNA sequence cloned from the genome of strain AY1, a member of subgroup 16SrI-B, revealed the presence of an open reading frame (ORF) that encodes a putative transposase (Tpase). The amino acid sequence of the Tpase shares a significant homology (an average of 36% identity and 55% similarity) with Tpases encoded by insertion sequences from several bacterial species of low G+C Gram-positive in the Firmicutes group. Six IS-like elements were cloned from both strain AY1 and NJAY (a member of subgroup 16SrI-A). All six clones contained one ORF encoding a putative Tpase with 322 amino acids. All six putative Tpases have an identical DDE motif that is most similar to the DDE consensus of the Tpases of the IS3 family. However, all six IS-like elements lack apparent terminal inverted repeat (IR) sequences. Southern blot hybridization analysis of EcoRI-digested genomic DNA of phytoplasma strains from both subgroups 16SrI-A and 16SrI-B revealed the presence of multiple copies of IS-like elements in each strain tested. The PCR assay using Tpase-specific primers revealed that six additional phytoplasma strains, representing six subgroups in the AY group, also contain IS-like elements. Substantial amino acid sequence variability suggested multiple species of Tpases or IS-like elements exist in the AY phytoplasma group.