Transposition without transposase: a spontaneous mutation in bacteria

Abstract

Transposition mutations are typically associated with the activities of transposable elements such as transposons and insertion sequences, whose mobility is dependent upon transposase enzymes that catalyze exchanges between element ends and target sites. We describe a single transposition event in which a block of donor sequence is inserted at a target site without the involvement of any known transposase or the ends of any known transposable element. We propose that this is a new type of spontaneous mutation which may be difficult to detect in standard mutant hunts but may be of evolutionary importance.Most spontaneous mutations can be sorted into four categories: base substitution, base addition or removal, and chromosomal rearrangements (deletions, duplications, inversions or transpositions). Rearrangements can arise by recombination between repeated sequences or can be catalyzed by transposases which act on conserved sequences at element ends. At low frequency, deletions and duplications can form without recombination between extensive repeats or transposases. Known bacterial transposition mutations are generated only by the activities of selfish elements.We describe here a spontaneous transposition event in Salmonella typhimurium that appears to have occurred without involvement of a transposable element or transposase. The mutation described moves a contiguous block of sequence (1.9 kb) and inserts it at a new site with no loss or duplication of the target sequence. While the system for detecting this mutant involved use of the transposable elements Tn10d(Tc) and Mud-lac, the mutational event itself occurred in strains devoid of any known transposase. Furthermore, the novel sequence junctions are not associated with the ends of any transposable element. Thus, the event may constitute a new type of spontaneous mutation. Rare events of this type may be difficult to detect without a specific selection system such as that used here. Spontaneous transposition events may have contributed to the evolution of hybrid proteins and multicistronic operons. MATERIALS AND METHODSBacterial strains. All strains are derivatives of S. typhimurium LT2 ( Table 1). The Tn10d(Tc) element is a transposition-defective derivative of transposon Tn10 lacking the transposase gene and the internal ends of IS10 (24). The MudJ element is a transposition-defective derivative of phage Mu described by Castilho et al. (3). Insertions of these elements in the eut operon have been described previously (18,20).Media. Complex medium was nutrient broth (NB) (0.8%; Difco Laboratories) supplemented with NaCl (0.5%). Minimal medium was Vogel and Bonner E medium (5) with added glucose (0.2%) as a carbon and energy source. Minimal E medium lacking citrate (NCE) (1) supplemented with 0.2% lactose was used in selecting mutants able to grow on lactose. The chromogenic ␤-galactosidase substrate 5-bromo-4-chloro-3-indoyl-␤-D-galactopyranoside (X-Gal) was added (final concentration, 20 g/ml) to solid NCE lactose medium to help visualize Lac ϩ colonies. An...

Genome-Wide Detection of Spontaneous Chromosomal Rearrangements in Bacteria

“…We classified the identified putative rearrangements into duplications, inversions and small deletions based on the relative chromosomal locations and orientations of the prefix and suffix in a read sampled across a putative rearrangement. One should note that the junctions caused by translocations would also be identified as duplication or inversion junctions, but since they are rare rearrangement events in S. typhimurium it is likely that translocations only had a small contribution in generating junction sequences in the chromosome [21] .…”

Essential Roles of RodA Peptidoglycan Polymerase and PBP2 Transpeptidase in Expression of Cell Wall-Spanning Supramolecular Organelles and ModulatingSalmonellaVirulence

“…Tn 10 d(Tc) is a 2.64 kbp transposon-defective mini transposon derived from Tn 10 , which requires the exogenous expression of a plasmid-borne transposase (135, 136). A P22 phage lysate library pooled from 12000 SL1344 Tn 10 d(Tc) transductant colonies was kindly provided for use in the mutagenesis screen (Hannah Spencer, Newcastle).…”