Department of Medical Microbiology and Immunology, University of Alberta, Edmonton, Canada T6G 2H71
Author for correspondence: Dennis Y. Kunimoto. Tel: +1 780 407 1418. Fax: +1 780 4927521. e-mail: Dennis.Kunimoto{at}Ualberta.Ca
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ABSTRACT |
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Keywords: mobile genetic elements, transposition, IS900, direct repeats, inverted repeats
Abbreviations: IS, insertion sequence; MAC, Mycobacterium avium complex
The GenBank accession number for the IS1626 sequence determined in this work is AF071067.
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INTRODUCTION |
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Insertion elements often show species specificity or a narrow host range. Only recently have insertion elements been described in mycobacteria and only five have been described from the Mycobacterium avium complex (MAC).
IS900, the first insertion sequence characterized in mycobacteria, was identified from the clone pMB22, which was derived from a genomic library prepared from a human Crohns disease isolate of Mycobacterium paratuberculosis (Green et al., 1989 ). IS900 was shown to be highly specific to M. paratuberculosis (Vary et al., 1990
; Moss et al., 1991
, 1992b
).
Some strains of M. avium were found to produce a complex banding pattern after low-stringency screening of Southern blots of M. avium strains using the clone pMB22 as a probe. Subsequently, IS901 was identified and found in most M. avium animal strains (Kunze et al., 1991 , 1992
; Nishimori et al., 1995
) and IS902 was identified from M. avium subsp. silvaticum (Moss et al., 1992a
). However, IS901 and IS902 are virtually identical.
Recently, IS1110 was discovered during a study of plasmid incidence in AIDS-derived M. avium strains. IS1110 is a 1457 bp element lacking terminal inverted repeats and is related to IS900 and IS901. IS1110 was detected in some M. avium isolates showing polymorphism and was considered to be a highly mobile genetic element since the transposition events of IS1110 can be detected in random colonies without any selection pressure (Hernandez Perez et al., 1994 ).
IS1245 was identified from an M. avium genomic fragment. It is a 1313 bp element with two imperfect inverted repeats and one ORF. IS1245 can be detected in a variety M. avium isolates, but human isolates characteristically show a high number of copies and a greater diversity of restriction fragment length patterns (Guerrero et al., 1995 ).
In Mycobacterium intracellulare, an insertion sequence, IS1141, has been identified (GenBank accession no. L10239). IS1141 is 1588 bp long, containing 23 bp inverted repeats at its ends. Although IS1141 only appears in some M. intracellulare strains, its transposition may be associated with colonial variation.
In this report, we describe another insertion sequence isolated from M. avium, IS1626, which is the most closely related insertion element to IS900 described to date.
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METHODS |
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DNA probes and colony screening.
The 300 bp HindIII/SacI fragment from pMB22/S12 of IS900 (a generous gift from Dr J. J. McFadden) was labelled with [-32P]dCTP by the random primer labelling method (Feinberg & Vogelstein, 1984
) and used as a probe for Southern blot analysis (Southern, 1975
) and for colony screening (see below). To clone the desired fragments, DNA fragments of the appropriate size were cut from a gel and isolated using a Geneclean II kit (Bio101) as described by the supplier. Fragments were then ligated into pBluescript (Stratagene) and recombinant plasmids were obtained after transformation of competent Escherichia coli DH5
cells. Colony screening (Grunstein & Hogness, 1975
) was used to select the IS900-related fragments.
DNA sequencing.
Plasmid DNA was prepared by a modified alkaline denaturation mini-preparation procedure (Sambrook et al., 1989 ). DNA was sequenced using the dideoxy chain-termination method of Sanger et al. (1977)
with either with the Sequenase version 2.0 DNA Sequencing Kit, (United States Biochemical) or by Taq DNA polymerase cycle sequencing as developed by Murray (1989)
.
T3/T7 primers were used to sequence the insert of pBluescript recombinants. Some primers were selected from newly determined sequences in order to sequence adjacent sections.
Computer analysis of sequence data.
DNA Strider 1.2 (Douglas, 1995 ) was used to check the repeats of IS1626 and determine the ORF. The Wisconsin GCG software (Genetics Computer Group) was used for pairwise comparison and multiple sequence alignment of the nucleotide sequence and predicted protein sequence of IS1626 with members of IS900 family
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RESULTS AND DISCUSSION |
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Strain 13219 was identified as M. intracellulare by Gen-Probe testing (GEN-PROBE), by 16S rRNA sequence determination (Rogall et al., 1990 ) and by restriction analysis of the 65 kDa heat-shock protein gene (Telenti et al., 1993
). It was negative by PCR for IS1245 and DT6, and was positive by PCR for the M. intracellulare specific fragment DT1 using the primers IN38 and IN41 (Thierry et al., 1993
). Results were as described in the literature for M. intracellulare and were the same as the ATCC M. intracellulare strain 13950. The IS in strain 13219 was not studied in detail. It is believed to be related, but not identical, to the IS in strain 21878.
As M. avium strain 21878 had three positive hybridizing bands, it was chosen for further study. From this strain, several subgenomic libraries were constructed with the desired size DNA corresponding to the three positive hybridizing bands, and then screened with a radiolabelled pMB22/S12 probe. Three corresponding clones, pB3.8, pB8.0 and pEH5.0, were recovered. Restriction mapping and Southern blotting confirmed that these three clones contained the same 1·3 kb fragment.
Identification of IS1626
The region containing the hybridizing fragment in pB3.8 was sequenced. Computer analysis of the sequence demonstrated a high degree of homology with IS900, indicating an IS900-related element. No inverted repeats or direct repeats were found. Therefore, from pB3.8 alone it was not possible to identify the exact end points of the IS900-related element. Further subcloning and sequencing of pB8.0 and pEH5.0 were subsequently performed. Comparison of the sequences of the three clones indicated that they contain the same fragment, which was designated IS1626. The divergence points of IS1626 for the three clones was determined.
To further confirm the termini of IS1626, DNA primers generated from sequences flanking IS1626 in pEH5.0 were used to directly sequence the equivalent insertion locus from two IS1626-free M. avium strains. By comparing the sequences at the divergence points from the three clones with the sequences from IS1626-free M. avium strains, the exact ends of IS1626 were determined. IS1626 is 1418 bp long and its G+C content is 65 mol%, which is similar to that found in the host genomic DNA (about 6270%). The G+C content of IS1626 is similar to that of IS elements IS900, IS901 and IS110, which also have G+C contents comparable to that of their hosts, M. paratuberculosis, M. avium and Streptomyces, respectively (Bruton & Chater, 1987 ; Green et al., 1989
; Kunze et al., 1991
).
In contrast to most other insertional elements, IS1626 does not possess either terminal repeats or flanking direct repeats. This absence is characteristic of previously characterized IS900-related elements. In most IS elements, the inverted repeats are believed to provide the recognition and binding sites for the transposase protein (Grindley, 1985 ; Berg et al., 1988
; Galas & Chandler, 1989
) and the direct repeats are most likely derived from the staggered cut of the targeted sequence followed by repair upon transposition. However, the structure of these IS900-related elements suggests that the inverted repeat is not essential for the binding of all transposases and that the target staggered cut is not the only model for transposition.
IS1626 insertion site
Sequence analysis of the flanking regions of clones pB3.8, pB8.0 and pEH5.0 and of the potential insertion sites in two IS1626-free M. avium isolates indicated an insertion site specificity. The consensus insertion site for IS1626 is 5'-CATGCN45*TCCTN2G-3' (the asterisk denotes site of insertion) and in three clones the element inserts in the same orientation with respect to this target sequence, which is similar to the insertion site sequences described for IS900-related elements. This specific insertion suggests that the flanking sequence plays an important role during transposition. Examination of 13 insertion site sequences from IS1626 and other IS900-related elements (Table 1) revealed that they all have CAT at the left insertion site and CCT at the right insertion site. These two 3 nt-elements were separated by 69 nt in the consensus insertion sequences. It was also noted that the same 3 nt elements also appear in their IS sequences with the internal CAT at the right side, 79 nt away from the CCT in the right flanking region, and the internal CCT at the left side, 511 nt away from the CAT in the left flanking region. These short sequences might be important for transposase recognition. The presence of nearby regions of limited homology with the ends of insertion sequences has been suggested to play a role in site selection (Galas & Chandler, 1989
). Morisato & Kleckner (1984)
found that the Tn10 transposase itself can make double-stranded cuts. Their study supports the cut-and-paste process. Therefore, it is conceivable that, like class I restriction endonucleases, the transposase in IS1626 and IS900-related IS elements may recognize and cut the CATCCT asymmetric sequences at both ends of transposable elements in the donor and the consensus insertion site in the recipient, and then ligate them in the same order. If these cleavages are made by blunt-end cutting followed by the cut-and-paste process, then simple insertion occurs. This transposition will not generate target duplication and the IS will always be in the same orientation with respect to their consensus site. At present, the mechanisms for transposition of IS900 family elements are far from completely understood. However, the insertion specificity of these IS elements may reflect base sequence recognition by their transposition proteins, and this recognition specificity may depend not only on the specific nucleotides but also the local DNA conformation.
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Sequence homologies between IS1626 and IS900, IS901, IS1110, IS110 and IS116 were determined by sequence alignment with the Wisconsin GCG software (Table 2). The DNA sequence comparisons indicated that IS1626 shows 82% homology with IS900, and a variable homology of 5362% with the other related IS elements. No significant homologies were found with IS elements from E. coli or other mycobacterial species. Comparison of the predicted amino acid sequences of the major ORFs from IS900, IS901, IS1110, IS110 and IS116 indicated that ORF1179 shows 63% identity with IS900, and 3042% identity with the others.
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Similar to IS900 and IS902, IS1626 has another long ORF, ORF930, on the complementary strand and in the reverse direction of ORF1179 (Fig. 2). ORF930 starts at the very 3' end of IS1626 at nt 1410 and ends at nt 481. Its ATG start codon is preceded by a ShineDalgarno sequence, AGGAGA, which is partially located in the flanking region. This ORF930 encodes a predicted protein of 310 amino acids. Its translational signals are formed from IS1626 and its flanking region, but it does not seem to have any upstream transcriptional signals. Furthermore, analysis of the putative insertion sites for IS900-related elements (Table 1
) showed that the complementary strands contain a ShineDalgarno sequence and an AUG-initiation-codon-like structure. It seems that these IS elements are in search of the translation and transcription signals needed to obtain an active element or to express genes encoded by the IS themselves. This may also explain their unusual insertion site specificity and the orientation of the element with respect to its target site. Comparison of the amino acid sequences indicated that ORF930 of IS1626 shows 53% identity to ORF2 of IS900 and 31% identity with ORF2 of IS902. However, computer searches of the GenBank database failed to reveal any significant similarity between ORF930 and other known sequences.
Distribution of IS1626 and evolution of IS900-related elements
Unlike other related IS elements, IS1626 seems to be uncommon in M. avium. Of 66 MAC isolates screened, only one M. avium strain contained the IS1626 element and only three copies were present. The source of IS1626 is unknown. As indicated by Kunze et al. (1991) , IS900 and IS901 might have been acquired by separate events after the speciation of their hosts, M. paratuberculosis and M. avium, respectively. Sequence analysis showed that there is 18% difference at the DNA level between IS1626 and IS900. This level of diversity is much higher than the genome sequence divergence estimated between M. avium and M. paratuberculosis (<2%) (McFadden et al., 1992
). The DNA diversity among the IS elements of M. avium is even higher. The sequence comparison between IS1626 and other IS900-related elements suggests that the acquisition of these elements was subsequent to the divergence of the host species. Since several related IS elements such as IS110, IS1110 and IS116 are carried on either a phage or a plasmid, phages and plasmids are probably the vehicles for these elements. With an association of these elements with such vectors, there may be high potential for horizontal transfer of IS900-like elements both within and among strains of MAC and other distantly related organisms. The distribution of IS900 family elements in Mycobacterium, Streptomyces and other bacteria, and the variable occurrence in M. avium strains may be evidence of horizontal transfer after the divergence of these species. This implies that horizontal transfer is common, but that the rate of transfer may be relatively low so that sufficient time passes for the IS element to accumulate substantial genetic divergence. The relatively few copies of IS1626 and its low frequency of occurrence in M. avium probably reflects the more recent invasion of this element in this species.
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Received 8 June 1999;
accepted 9 July 1999.