Construction and use of integrative vectors to express foreign genes in mycobacteria

Electroporation of Mycobacteria

The introduction of DNA into bacterial cells is one of the foundational methods of bacterial genetics. Transformation of mycobacterial species is complicated due to the structure of the cell wall, which has a complex outer layer with low permeability. Electroporation has become a routine procedure in genetic studies. In this process, cells are subjected to a brief high-voltage electrical impulse which allows the entry of DNA. It can be used to introduce plasmid DNA, phage DNA, or oligonucleotides. This chapter presents methods for introducing DNA into a representative slow-growing species, M. tuberculosis, and a representative fast-growing species, M. smegmatis. Other mycobacteria can be transformed using variations of these methods, although the efficiency of transformation will vary.

Stable Expression of Lentiviral Antigens by Quality-Controlled Recombinant Mycobacterium bovis BCG Vectors

The well-established safety profile of the tuberculosis vaccine strain, Mycobacterium bovis bacille Calmette-Guérin (BCG), makes it an attractive vehicle for heterologous expression of antigens from clinically relevant pathogens. However, successful generation of recombinant BCG strains possessing consistent insert expression has encountered challenges in stability. Here, we describe a method for the development of large recombinant BCG accession lots which stably express the lentiviral antigens, human immunodeficiency virus (HIV) gp120 and simian immunodeficiency virus (SIV) Gag, using selectable leucine auxotrophic complementation. Successful establishment of vaccine stability stems from stringent quality control criteria which not only screen for highly stable complemented BCG ΔleuCD transformants but also thoroughly characterize postproduction quality. These parameters include consistent production of correctly sized antigen, retention of sequence-pure plasmid DNA, freeze-thaw recovery, enumeration of CFU, and assessment of cellular aggregates. Importantly, these quality assurance procedures were indicative of overall vaccine stability, were predictive for successful antigen expression in subsequent passaging both in vitro and in vivo, and correlated with induction of immune responses in murine models. This study has yielded a quality-controlled BCG ΔleuCD vaccine expressing HIV gp120 that retained stable full-length expression after 10(24)-fold amplification in vitro and following 60 days of growth in mice. A second vaccine lot expressed full-length SIV Gag for >10(68)-fold amplification in vitro and induced potent antigen-specific T cell populations in vaccinated mice. Production of large, well-defined recombinant BCG ΔleuCD lots can allow confidence that vaccine materials for immunogenicity and protection studies are not negatively affected by instability or differences between freshly grown production batches.

Electroporation of mycobacteria

High-efficiency transformation of DNA is integral to the study of mycobacteria, allowing genetic manipulation. Electroporation is the most widely used method for introducing DNA into mycobacterial strains. Many parameters contribute to high-efficiency transformation; these include the species per strain, the transforming DNA, the selectable marker, the growth medium additives, and the conditions of electroporation. In this chapter we provide an optimized method for the transformation of representative slow- and fast-growing species of mycobacteria-Mycobacterium tuberculosis and M. smegmatis, respectively.

Recombinant Mycobacterium bovis BCG

The Bacillus Calmette-Guerin (BCG) is an attenuated strain of Mycobacterium bovis that has been broadly used as a vaccine against human tuberculosis. This live bacterial vaccine is able to establish a persistent infection and induces both cellular and humoral immune responses. The development of mycobacterial genetic systems to express foreign antigens and the adjuvanticity of BCG are the basis of the potential use of this attenuated mycobacterium as a recombinant vaccine. Over the years, a range of strategies has been developed to allow controlled and stable expression of viral, bacterial and parasite antigens in BCG. Herein, we review the strategies developed to express heterologous antigens in BCG and the immune response elicited by recombinant BCG constructs. In addition, the use of recombinant BCG as an immunomodulator and future perspectives of BCG as a recombinant vaccine vector are discussed.

Electroporation of mycobacteria

High-efficiency transformation is a major limitation in the study of mycobacteria. The genus Mycobacterium can be difficult to transform; this is mainly caused by the thick and waxy cell wall but is compounded by the fact that most molecular techniques have been developed for distantly related species such as Escherichia coli and Bacillus subtilis. In spite of these obstacles, mycobacterial plasmids have been identified, and DNA transformation of many mycobacterial species has now been described. The most successful method for introducing DNA into mycobacteria is electroporation. Many parameters contribute to successful transformation; these include the species/strain, the nature of the transforming DNA, the selectable marker used, the growth medium, and the conditions for the electroporation pulse. Optimized methods for the transformation of both slow-grower and fast-grower are detailed here. Transformation efficiencies for different mycobacterial species and with various selectable markers are reported.

Auxotrophic complementation as a selectable marker for stable expression of foreign antigens in Mycobacterium bovis BCG

Mycobacterium bovis BCG has the potential to be an effective live vector for multivalent vaccines. However, most mycobacterial cloning vectors rely on antibiotic resistance genes as selectable markers, which would be undesirable in any practical vaccine. Here we report the use of auxotrophic complementation as a selectable marker that would be suitable for use in a recombinant vaccine. A BCG auxotrophic for the amino acid leucine was constructed by knocking out the leuD gene by unmarked homologous recombination. Expression of leuD on a plasmid not only allowed complementation, but also acted as a selectable marker. Removal of the kanamycin resistance gene, which remained necessary for plasmid manipulations in Escherichia coli, was accomplished by two different methods: restriction enzyme digestion followed by re-ligation before BCG transformation, or by Cre-loxP in vitro recombination mediated by the bacteriophage P1 Cre Recombinase. Stability of the plasmid was evaluated during in vitro and in vivo growth of the recombinant BCG in comparison to selection by antibiotic resistance. The new system was highly stable even during in vivo growth, as the selective pressure is maintained, whereas the conventional vector was unstable in the absence of selective pressure. This new system will now allow the construction of potential recombinante vaccine strains using stable multicopy plasmid vectors without the inclusion of antibiotic resistance markers.

Factors influencing the immune response to foreign antigen expressed in recombinant BCG vaccines

A wide range of recombinant BCG vaccine candidates containing foreign viral, bacterial, parasite or immunomodulatory genetic material have been developed and evaluated, primarily in animal models, for immune response to the foreign antigen. This review considers some of the factors that may influence the immunogenicity of these vaccines. The influence of levels and timing of expression of the foreign antigen and the use of targeting sequences are considered in the first section. Genetic and functional stability of rBCG is reviewed in the second section. In the last section, the influence of dose and route of immunization, strain of BCG and the animal model used are discussed.

Immune response of pigs inoculated with Mycobacterium bovis BCG expressing a truncated form of GP5 and M protein of porcine reproductive and respiratory syndrome virus

Pigs were immunised with recombinant BCG (rBCG) expressing a truncated form of GP5 (lacking the first 30 NH(2)-terminal residues) (rBCGGP5) and M protein (rBCGM) of porcine reproductive and respiratory syndrome virus (PRRSV). At 30 days post-inoculation (dpi), pigs inoculated with rBCGGP5 and rBCGM developed a specific humoral immune response against the viral proteins, as detected by commercial ELISA and Western blot tests, and at 60 dpi, three out of five animals developed neutralizing antibodies with titers ranging from 1:4 to 1:8. At 67 dpi, an IFN-gamma response against BCG antigens, but not against the viral proteins, was detected by ELISPOT in inoculated pigs. Following challenge with a pathogenic strain of PRRSV, pigs inoculated with rBCG showed lower (P<0.05) temperature, viremia and virus load in bronchial lymph nodes than control animals, suggesting the establishment of partial protection against PRRSV infection.

Construction and immunogenicity of recombinant Mycobacterium bovis BCG expressing GP5 and M protein of porcine reproductive respiratory syndrome virus

Mycobacterium bovis BCG was used to express a truncated form of GP5 (lacking the first 30 NH(2)-terminal residues) and M protein of porcine reproductive and respiratory syndrome virus (PRRSV). The PRRSV proteins were expressed in BCG under control of the mycobacterial hsp60 gene promoter either in the mycobacterial cytoplasm (BCGGP5cyt and BCGMcyt) or as MT19-fusion proteins on the mycobacterial surface (BCGGP5surf and BCGMsurf). Mice inoculated with BCGGP5surf and BCGMsurf developed antibodies against the viral proteins at 30 days post-inoculation (dpi) as detected by ELISA and Western blot. By 60 dpi, the animals developed titer of neutralizing antibodies of 8. A PRRSV-specific gamma interferon response was also detected in splenocytes of recombinant BCG-inoculated mice at 60 and 90 dpi. These results indicate that BCG was able to express antigens of PRRSV and elicit an immune response against the viral proteins in mice.

Immunization with a portion of rickettsial outer membrane protein A stimulates protective immunity against spotted fever rickettsiosis

Two approaches for presentation of a part of the rickettsial outer membrane protein A (OmpA) of Rickettsia rickettsii, namely (1) recombinant Mycobacterium vaccae (rMV) or (2) recombinant DNA vaccine, stimulated protective immunity against a lethal challenge with the closely related bacterium, R. conorii, in mice. After primary immunization with rMV and booster immunization with homologous recombinant protein, 67 and 55% of mice were protected against challenge in two experiments. DNA vaccination with booster recombinant protein immunization protected six out of eight animals from a lethal challenge. Production of IFN-gamma by antigen-exposed T-lymphocytes of DNA vaccine recipients indicated that cellular immunity had been stimulated.

Expression of the B-cell and T-cell epitopes of the rabies virus nucleoprotein in Mycobacterium bovis BCG and induction of an humoral response in mice

Expression vectors containing rabies virus nucleoprotein B-cell and T-cell epitopes in Mycobacterium bovis BCG were constructed. The epitopes were subcloned into the M. leprae 18-kDa gene to ensure correct presentation to the host immune system. Expression of the 18-kDa::B+T epitope fusion protein was driven by either the hsp60 promoter, which is constitutively activated at a high level in M. bovis BCG, or the 18-kDa promoter, which is strongly induced in vivo. Mice were immunised intra-peritoneally with the recombinant BCG cultures and compared to a control group vaccinated with the commercial rabies vaccine Rai-SAD. Both of the expression vectors elicited a higher antibody titre than that of the rabies vaccine, with the highest response shown by M. bovis BCG (pUP203), expression controlled by the 18-kDa promoter. Immunisation with M. bovis BCG (pUP202), expression controlled by the hsp60 promoter, resulted in a continuously increasing antibody titre up to 60 days post immunisation. The mice antibodies were also capable of recognising the whole rabies virus and not only the synthetic peptide epitopes.

Mycobacterium avium subsp. paratuberculosis in Veterinary Medicine

Mycobacterium avium subsp. paratuberculosis (basonym M. paratuberculosis) is the etiologic agent of a severe gastroenteritis in ruminants known as Johne's disease. Economic losses to the cattle industry in the United States are staggering, reaching $1.5 billion annually. A potential pathogenic role in humans in the etiology of Crohn's disease is under investigation. In this article, we review the epidemiology, pathogenesis, diagnostics, and disease control measures of this important veterinary pathogen. We emphasize molecular genetic aspects including the description of markers used for strain identification, diagnostics, and phylogenetic analysis. Recent important advances in the development of animal models and genetic systems to study M. paratuberculosis virulence determinants are also discussed. We conclude with proposals for the applications of these models and recombinant technology to the development of diagnostic, control, and therapeutic measures.

Isolation of a novel insertion sequence from Mycobacterium fortuitum using a trap vector based on inactivation of a lacZ reporter gene

An insertion sequence of Mycobacterium fortuitum has been isolated using a trap vector following insertion in and inactivation of the lacZ reporter gene. The trap vector is a temperature-sensitive (ts) Escherichia coli-mycobacterium shuttle plasmid, pCD4, which contains ts oriM, the kanamycin-resistance gene as a selection marker and a lacZ expression cassette. The ts mutation present in pCD4 functions in mycobacteria and enables screening for transposable elements from the mycobacterial genome that disrupt the lacZ gene by screening for white colonies on X-Gal plates in both mycobacterial as well as E. coli hosts. The vector was used to isolate a novel 1.653 kb insertion sequence from M. fortuitum named IS219. IS219 duplicated host DNA at the target site, had inverted repeats at its ends and contained two ORFs on one strand. One of the predicted proteins showed homology to a putative transposase from Acetobacter pasteurianus. IS219 was present in two copies in the genome of M. fortuitum. The trap vector appears to be useful in trapping insertion sequences from different mycobacteria by screening for the disrupted LacZ phenotype.

Mycobacteria: bugs and bugbears (two steps forward and one step back)

The use of molecular techniques to study the mycobacteria has advanced greatly since the first genomic libraries of Mycobacterium tuberculosis and M. leprae were constructed in 1985. However, there are still pitfalls for the unwary. Most of the problems associated with the use of molecular techniques to study mycobacteria can be related to one of the following problems: slow growth rate causing problems with contamination; the formation of macroscopic clumps when grown in culture; resistance to standard chemical lysis procedures; the requirement for containment facilities for pathogenic species; the lack of suitable genetic vectors; and the problems of spontaneous antibiotic resistance. Despite these problems, considerable progress has been made and standard techniques have been developed for the preparation of protein, nucleic acids (DNA and RNA) and cell wall components, chemical and transposon mutagenesis and gene replacement methods, the use of reporter genes and expression vectors, and improved detection and drug sensitivity testing.

Context-sensitive transposition of IS6110 in mycobacteria

The rational use of IS6110 fingerprinting for studies of the molecular epidemiology and evolution of Mycobacterium tuberculosis requires understanding of the dynamics of transposition. In laboratory model systems, it has been shown that transposition is context-sensitive, i.e. it is influenced by the nature of the site in which the insertion sequence is presented. Stimulation of transposition by activation of an adjacent promoter supports the hypothesis that transposition occurs more readily from transcriptionally active locations. In addition, it has been shown that transposition can be enhanced by the expression of the transposase in trans. These findings imply that the frequency of transposition will vary substantially between different strains of M. tuberculosis, and furthermore that a hitherto stable strain may develop more rapid variation due to transposition into an active site. The use of IS6110 fingerprinting for the analysis of longer-range relationships between M. tuberculosis isolates therefore needs to be interpreted with care.

Stimulation of transposition of the Mycobacterium tuberculosis insertion sequence IS6110 by exposure to a microaerobic environment

The Mycobacterium tuberculosis-specific insertion sequence IS6110/986 has been widely used as a probe because of the multiple polymorphism observed among different strains. To investigate transposition of IS6110, a series of artificially constructed composite transposons containing IS6110 and a kanamycin resistance marker were constructed. The composite transposons were inserted into a conditionally replicating, thermosensitive, Escherichia coli-mycobacterial shuttle vector and introduced into M. smegmatis mc2155. Lawns of transformants were grown at the permissive temperature on kanamycin-supplemented agar and subsequently prevented from further growth by shifting to the non-permissive temperature. Under normal atmospheric conditions, kanamycin-resistant papillae appeared after only about 5-6 weeks of incubation. However, these events were not associated with transposon mobilization. In contrast, lawns that were exposed to a 48 h microaerobic shock generated kanamycin-resistant papillae after only 6-14 days. These events were generated by conservative transposition of the IS6110 composite transposon into the M. smegmatis chromosome, with loss of the shuttle vector. In common with other IS3 family elements, transposition of IS6110 is thought to be controlled by translational frameshifting. However, we were unable to detect any significant frameshifting within the putative frameshifting site of IS6110, and the level of frameshifting was not affected by microaerobic incubation. The finding that transposition of IS6110 is stimulated by incubation at reduced oxygen tensions may be relevant to transposition of IS6110 in M. tuberculosis harboured within TB lesions.

Methanotrophs, Methylosinus trichosporium OB3b, sMMO, and their application to bioremediation

One of the most problematic groups of the USEPA and EU priority pollutants are the halogenated organic compounds. These substances have a wide range of industrial applications, such as solvents and cleaners. Inadequate disposal techniques and accidental spillages have led to their detection in soil, groundwater, and river sediments. Persistence of these compounds in the environment has resulted from low levels of biodegradation due to chemical structural features that preclude or retard biological attack. Research has indicated the idea that treatment systems based on methanotrophic co-metabolic transformation may be a cost-effective and efficient alternative to physical methods because of the potential for high transformation rates, the possibility of complete compound degradation without the formation of toxic metabolites, applicability to a broad spectrum of compounds, and the use of a widely available and inexpensive growth substrate. A substantial amount of work concerning methanotrophic cometabolic transformations has been carried out using the soluble form of methane monooxygenase (sMMO) from the obligate methanotroph Methylosinus trichosporium OB3b. This NADH-dependent monooxygenase is derepressed when cells are grown under copper stress. sMMO has a wider specificity than the particulate form. sMMO has been shown to degrade trichloroethylene (TCE) at a rate of at least one order of magnitude faster than obtained with other mixed and pure cultures, suggesting it has a wider application to bioremediation. Furthermore, sMMO catalyzes an unusually wide range of oxidation reactions, including the hydroxylation of alkanes, epoxidation of alkenes, ethers, halogenated methanes, cyclic and aromatic compounds including compounds, that are resistant to degradation in the environment. However, the practical application of methantrophs and Methylosinus trichosporium OB3b to the treatment of chlorinated organics has met with mixed success. Although oxidation rates are rapid, compound oxidation with M. trichosporium OB3b is difficult. This fastidious organism grows relatively slowly, which limits the speed with which sMMO expressing biomass can be generated. Furthermore, product toxicity toward the cell, affecting the stability of the enzyme when transforming certain compounds has been observed, for example, by the products of 1,2,3 trichlorobenzene hydroxylation (2,3,4- and 3,4,5-trichlorophenol) and of TCE degradation (chloral hydrate). Because of this toxicity and the inability of sMMO to further oxidize its own hydroxylation products, the ability of methane monoxygenase to carry out the monooxygenation of a wide variety of substituted aromatics and polyaromatics cannot be fully exploited in M. trichosporium OB3b. Many of these problems could be overcome by the use of either a mixed downstream heterotrophic population of organisms that could accommodate the products of hydroxylation or to express sMMO in an organism that could metabolize the products of hydroxylation. The latter of these two approaches would have several advantages. The main benefit would be the removal of the need for methane, which is required to induce sMMO in M. trichosporium OB3b, and supply carbon and energy to the cells that continuously oxidise the target compound, but also acts as a competitive inhibitor of sMMO. Instead, the recombinant could utilize the products of sMMO-mediated hydroxylation as a carbon source.

IS990, a new species-specific insertion-sequence-related element of the Mycobacterium tuberculosis complex

The structure and distribution of 1S990, a new Mycobacterium tuberculosis DNA sequence with homology to characterized insertion sequences (ISs), were investigated. IS990 was related to IS elements of the IS3 family and was present as a single copy in all 21 investigated M. tuberculosis strains, two Mycobacterium bovis strains and two M. bovis BCG strains. The sequence appears to be specific for the M. tuberculosis complex. The element carries two frameshift mutations and appears to be defective.

The genetic reconstruction of BCG as a new immunotherapeutic tool

The bacillus of Calmette and Guérin (BCG), long appreciated for its role as a live vaccine for the prevention of tuberculosis, is undergoing a rebirth as a recombinant delivery vehicle for foreign antigens and bioactive proteins. Recombinant BCG causes long-lived specific humoral and cellular immunity and may ultimately prove to be a powerful and cost-effective new weapon against both infectious pathogens and certain cancers.

IS900 targets translation initiation signals in Mycobacterium avium subsp. paratuberculosis to facilitate expression of its hed gene

The Mycobacterium avium subsp. paratuberculosis (formerly Mycobacterium paratuberculosis) atypical insertion sequence, IS900, encodes a novel gene on the complementary strand to the putative transposase, p43. This gene requires a promoter, ribosome binding site (RBS) and termination codon to be acquired upon insertion into the M. avium subsp. paratuberculosis genome and hence is designated the hed (host expression-dependent) gene of IS900. Analysis of IS900 insertion sites suggests that this element targets translation initiation signals in M. avium subsp. paratuberculosis, specifically inserting between the RBS and start codon of a putative gene sequence. This aligns the hed initiation codon adjacent to a functional RBS and possibly downstream of an active promoter, driving expression of Hed protein. We have confirmed this unique targeting process by detecting expression of hed in M. avium subsp. paratuberculosis at the level of transcription by reverse transcription-PCR. Further, two Hed-specific antibodies detected Hed translation products in Western blots of protein extracts from M. avium subsp. paratuberculosis. A recombinant form of Hed expressed and purified from Escherichia coli will facilitate studies of IS900 transposition and will also be assessed as a diagnostic antigen for M. avium subsp. paratuberculosis disease. Implications of IS900 insertion in M. avium subsp. paratuberculosis pathogenicity are discussed.

Techniques for genetic engineering in mycobacteria. Alternative host strains, DNA-transfer systems and vectors

The study of mycobacterial genetics has experienced quick technical developments in the past ten years, despite a relatively slow start, caused by difficulties in accessing these recalcitrant species. The study of mycobacterial pathogenesis is important in the development of new ways of treating tuberculosis and leprosy, now that the emergence of antibiotic-resistant strains has reduced the effectiveness of current therapies. The tuberculosis vaccine strain M. bovis BCG might be used as a vector for multivalent vaccination. Also, non-pathogenic mycobacterial strains have many possible biotechnological applications. After giving a historical overview of methods and techniques, we will discuss recent developments in the search for alternative host strains and DNA transfer systems. Special attention will be given to the development of vectors and techniques for stabilizing foreign DNA in mycobacteria.

Activity of mycobacterial promoters during intracellular and extracellular growth

pUS933, a bifunctional Mycobacterium-Escherichia coli translational fusion vector containing an amino-terminally truncated E. coli lacZ reporter gene, was constructed. Derivatives of pUS933, containing the promoter, RBS and start codon of the Mycobacterium bovis BCG hsp60 gene, the Mycobacterium leprae 28 kDa gene and the M. leprae 18 kDa gene were constructed and introduced into E. coli, Mycobacterium smegmatis and M. bovis BCG. beta-Galactosidase activity was measured for mycobacteria grown in liquid culture. Primer-extension analysis was used to determine the transcriptional start point for the 18 kDa promoter in M. smegmatis. Murine macrophages were infected with recombinant BCG containing the pUS933 derivatives and expression levels were examined, by fluorescence microscopy and fluorometry, during intracellular growth of BCG. Both the BCG hsp60 gene promoter and the M. leprae 28 kDa gene promoter gave high levels of beta-galactosidase expression in all situations examined. In contrast, the M. leprae 18 kDa promoter fragment gave very low levels of expression in M. smegmatis and BCG grown in liquid culture, but in BCG growing within macrophages it was induced to levels almost as high as the other promoters. This indicated that the 18 kDa gene is specifically activated during intracellular growth and may therefore be involved in survival of M. leprae within macrophages. This pattern of regulation may be useful for controlling expression of foreign genes in recombinant BCG strains.

Gene replacement by homologous recombination in Mycobacterium bovis BCG

Gene replacement by homologous recombination is a powerful tool for fundamental studies of gene function, as well as allowing specific attenuation of pathogens, but has proved difficult to achieve for Mycobacterium tuberculosis. We have used a plasmid-based test system to demonstrate the occurrence of homologous recombination in the tuberculosis vaccine strain Mycobacterium bovis BCG, and we have successfully replaced a target gene in BCG by homologous recombination, using a shuttle plasmid. Specific inactivation of selected genes will facilitate study of virulence factors and drug resistance as well as allowing rational attenuation of M. tuberculosis for the production of new vaccines.

Analysis of the Mycobacterium tuberculosis 85A antigen promoter region

A mycobacterial expression-secretion vector was constructed in which the Escherichia coli alkaline phosphatase (phoA) reporter gene was placed under the control of the Mycobacterium tuberculosis 85A promoter and secretion signal sequences. In recombinant Mycobacterium smegmatis and Mycobacterium bovis BCG, PhoA activity could readily be detected on the mycobacterial cell surface and in the culture supernatant, indicating that the 85A signals can drive heterologous expression and secretion in both species. In contrast to the mycobacteria, the 85A promoter did not function in E. coli. We mapped the promoter region by progressive deletions using BAL 31 exonuclease and by primer extension analysis. Insertion and deletion mutations within the promoter region indicated that, unlike most E. coli promoters but similar to Streptomyces promoters, the position of the putative -35 region was not critical for efficient promoter activity. In addition, we investigated the ability of the identified signals to drive the production and secretion in BCG of recombinant Schistosoma mansoni glutathione S-transferase (Sm28GST), a protective antigen against schistosomiasis. BALB/c mice immunized with the recombinant BCG by a single dose exhibited a weak but specific T-cell response to Sm28GST.

Characterization of IS1110, a highly mobile genetic element from Mycobacterium avium

A highly mobile insertion sequence designated IS1110 was detected in Mycobacterium avium strain LR541 following an observed increase in size of the plasmid pLR20. Genomic libraries of M. avium strains carrying either parental pLR20 or the modified plasmid (pLR20') were constructed and the sequence of the relevant clones was determined to characterize the insertion sequence and the target region. IS1110 is a 1457 bp element lacking terminal inverted repeats, and is related to IS900 (from Mycobacterium paratuberculosis), IS901 and IS902 (from M. avium) and to IS116 (from Streptomyces clavuligerus). LR541 carries several copies of IS1110. Individual colonies from the same plate show differences in Southern blot patterns when tested with an IS1110-derived probe; the ability to detect transposition events in random colonies, without any selection pressure, indicates an exceptionally high degree of mobility, which will be invaluable for transposon mutagenesis. Analyses of M. avium isolates from human, veterinary, and environmental sources showed that IS1110-hybridizing sequences are present in some M. avium isolates but they were not detected in strains of other mycobacterial species. The polymorphism exhibited in M. avium isolates suggests that this element may be useful for molecular epidemiological studies of M. avium infections.