Variation of Serine-Aspartate Repeats in Membrane Proteins Possibly Contributes to Staphylococcal Microevolution

Tandem repeats (either as microsatellites or minisatellites) in eukaryotic and prokaryotic organisms are mutation-prone DNA. While minisatellites in prokaryotic genomes are underrepresented, the cell surface adhesins of bacteria often contain the minisatellite SD repeats, encoding the amino acid pair of serine-asparatate, especially in Staphylococcal strains. However, their relationship to biological functions is still elusive. In this study, effort was made to uncover the copy number variations of SD repeats by bioinformatic analysis and to detect changes in SD repeats during a plasmid-based assay, as a first step to understand its biological functions. The SD repeats were found to be mainly present in the cell surface proteins. The SD repeats were genetically unstable and polymorphic in terms of copy numbers and sequence compositions. Unlike SNPs, the change of its copy number was reversible, without frame shifting. More significantly, a rearrangement hot spot, the ATTC/AGRT site, was found to be mainly responsible for the instability and reversibility of SD repeats. These characteristics of SD repeats may facilitate bacteria to respond to environmental changes, with low cost, low risk and high efficiency

Sequence Diversities of Serine-Aspartate Repeat Genes among Staphylococcus aureus Isolates from Different Hosts Presumably by Horizontal Gene Transfer

BACKGROUND: Horizontal gene transfer (HGT) is recognized as one of the major forces for bacterial genome evolution. Many clinically important bacteria may acquire virulence factors and antibiotic resistance through HGT. The comparative genomic analysis has become an important tool for identifying HGT in emerging pathogens. In this study, the Serine-Aspartate Repeat (Sdr) family has been compared among different sources of Staphylococcus aureus (S. aureus) to discover sequence diversities within their genomes. METHODOLOGY/PRINCIPAL FINDINGS: Four sdr genes were analyzed for 21 different S. aureus strains and 218 mastitis-associated S. aureus isolates from Canada. Comparative genomic analyses revealed that S. aureus strains from bovine mastitis (RF122 and mastitis isolates in this study), ovine mastitis (ED133), pig (ST398), chicken (ED98), and human methicillin-resistant S. aureus (MRSA) (TCH130, MRSA252, Mu3, Mu50, N315, 04-02981, JH1 and JH9) were highly associated with one another, presumably due to HGT. In addition, several types of insertion and deletion were found in sdr genes of many isolates. A new insertion sequence was found in mastitis isolates, which was presumably responsible for the HGT of sdrC gene among different strains. Moreover, the sdr genes could be used to type S. aureus. Regional difference of sdr genes distribution was also indicated among the tested S. aureus isolates. Finally, certain associations were found between sdr genes and subclinical or clinical mastitis isolates. CONCLUSIONS: Certain sdr gene sequences were shared in S. aureus strains and isolates from different species presumably due to HGT. Our results also suggest that the distributional assay of virulence factors should detect the full sequences or full functional regions of these factors. The traditional assay using short conserved regions may not be accurate or credible. These findings have important implications with regard to animal husbandry practices that may inadvertently enhance the contact of human and animal bacterial pathogens

Characterization of a functional insertion sequence ISSau2 from Staphylococcus aureus

Abstract Background ISSau2 has been suggested as a member of the IS150 f subgroup in the IS3 family. It encodes a fusion transposase OrfAB produced by programmed − 1 translational frameshifting with two overlapping reading frames orfA and orfB. To better characterize ISSau2, the binding and cleaving activities of the ISSau2 transposase and its transposition frequency were studied. Results The purified ISSau2 transposase OrfAB was a functional protein in vitro since it bound specifically to ISSau2 terminal inverted repeat sequences (IRs) and cleaved the transposon ends at the artificial mini-transposon pUC19-IRL-gfp-IRR. In addition, the transposition frequency of ISSau2 in vivo was approximately 1.76 ± 0.13 × 10− 3, based on a GFP hop-on assay. Furthermore, OrfB cleaved IRs with the similar catalytic activity of OrfAB, while OrfA had no catalytic activity. Finally, either OrfA or OrfB significantly reduced the transposition of ISSau2 induced by OrfAB. Conclusion We have confirmed that ISSau2 is a member of IS150/IS3 family. The ISSau2 transposase OrfAB could bind to and cleave the specific fragments containing the terminal inverted repeat sequences and induce the transposition, suggesting that ISSau2 is at least partially functional. Meanwhile, both OrfA and OrfB inhibited the transposition by ISSau2. Our results will help understand biological roles of ISSau2 in its host S. aureus

Characterization of Insertion Sequence ISSau2 in the Human and Livestock-Associated Staphylococcus aureus.

Mobile genetic elements play important roles in evolution and diversification of bacterial genomes. ISSau2 is 1660bp in length with terminal 5'-TG and CA-3' dinucleotides and has two overlapping reading frames orfA and orfB. It has been found in a wide range of S. aureus, such as HA-MRSA252, LGA251, MRSA S0385 and ED133. To determine distribution of ISSau2, 164 S. aureus isolates from milk samples of mastitic cows from our laboratory and all the S. aureus strains from the National Center for Biotechnology Information (NCBI) database were screened for the presence of ISSau2. Next, in order to explore a potential relationship among S. aureus ISSau2-containing strains and isolates, a relationship among 10 ISSau2-positive S. aureus isolates and 27 ISSau2-positive S. aureus strains was investigated by a phylogenetic analysis. These ISSau2 isolates and strains could be classified into four groups (A, B, C and D). The strains or isolates in Group D were all isolated from mammary glands, suggesting tissue specificity. All strains in Group B had an identical ISSau2 derivative, termed ISSau21628, with 32bp deletion at the 3' terminus. ISSau21628 in strain ST398 from Group B was closely related to ISSau2 in strain LGA251 from Group D