Summary auto-generated
This study characterizes type III secretion (TTS) gene clusters in Burkholderia pseudomallei, the causative agent of melioidosis, and related Burkholderia species. The researchers sequenced the TTS1 gene cluster in B. pseudomallei strain E503, finding it organized similarly to TTS systems in plant pathogens Ralstonia solanacearum and Xanthomonas species. The cluster contains genes encoding structural secretion proteins and several open reading frames (ORFs) likely encoding secreted effector proteins. RT-PCR analysis confirmed that TTS genes are expressed at 37°C in broth culture. Using genome sequence data and PCR assays, the authors identified a second TTS gene cluster (TTS2) present in both B. pseudomallei and B. mallei with 99% sequence identity, but apparently absent from B. mallei's TTS1 region. PCR analysis revealed TTS2 is also present in the avirulent B. thailandensis. Although TTS1 is absent from most B. thailandensis strains (except one exception, E27), TTS2 is present across all three species. The two TTS systems show similar gene organization but sufficient sequence divergence suggesting distinct functional roles in bacterial pathogenesis.
Key findings
- TTS1 gene cluster in B. pseudomallei is structurally similar to plant pathogen TTS systems but contains unique putative effector genes downstream of the core secretion genes
- TTS1 genes are expressed at 37°C in laboratory culture and are present in virulent B. pseudomallei strains but absent from most avirulent B. thailandensis strains
- A second type III secretion cluster (TTS2) with 99% identity exists in both B. pseudomallei and B. mallei, and is present in all three Burkholderia species tested
- The two TTS systems (TTS1 and TTS2) are sufficiently divergent in nucleotide sequence to suggest they may have different biological roles in pathogenesis
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Abstract
Burkholderia pseudomallei, the causative agent of melioidosis, carries a cluster of genes closely related in organisation to the type III secretion (TTS) system gene clusters of the plant pathogens Ralstonia solanacearum and Xanthomonas spp. The TTS gene cluster (TTS1) is present only in B. pseudomallei and not in avirulent B. thailandensis. Adjacent to the gene cluster encoding putative secreton structural proteins lie a number of open reading frames (ORFs) encoding putative proteins with little or no homology to known proteins, with the exception of one predicted protein with homology to Pseudomonas syringae HrpK. In both R. solanacearum and Xanthomonas spp., genes in this location encode secreted effector proteins. RT-PCR analysis indicated that TTS genes, including two of these ORFs, are expressed in broth at 37°C. Analysis of genome sequence data identified a second cluster of TTS genes (TTS2) present in both B. pseudomallei and B. mallei (99% identity). However, B. mallei appears to lack the TTS1 gene cluster. PCR assays indicated that TTS2 was also present in B. thailandensis. TTS1 and TTS2 are similar in gene organisation, but nucleotide sequences are sufficiently divergent to suggest that the two TTS systems may have different roles.