The phage Mu repressor c and IS30 transposase proteins are significantly related

Analysis of the N-terminal DNA binding domain of the IS30 transposase

IS30 is the founding member of a large family of widely spread bacterial insertion sequences with closely related transposases. The N-terminal end of the IS30 transposase had been shown to retain sequence-specific DNA binding activity and to protect the IS30 terminal inverted repeats. Structural predictions revealed the presence of a helix-helix-turn-helix motif (H-HTH2) which, in the case of IS30, is preceded by an additional helix-turn-helix motif (HTH1). Analysis of deletion and point mutants in this region revealed that both motifs are important for IS30 transposition. IS30 exhibits two types of insertion specificity preferring either a 24 bp palindromic hot-spot (GOHS) or sequences resembling its ends [left and right terminal inverted repeat (IRL and IRR)]. Results are presented suggesting that the HTH1 region is required for GOHS targeting and interferes with the inverted repeat (IR) targeting. On the other hand, H-HTH2 appears to be required for both. The binding activities of the mutant proteins to the terminal IS30 IRs as measured by gel retardation correlated well with these results. Furthermore, close inspection of the H-HTH2 region revealed significant amino acid identity with a similar predicted secondary structure carried by the transcriptional regulator FixJ of Sinorhizobium meliloti and involved in FixJ binding to its target sequence. This suggests that FixJ and IS30 transposase share similar sequence-specific DNA binding mechanisms.

Mu transposase-stimulated illegitimate recombination of Tn3kan- and IS101-containing plasmids

The transposable bacteriophage Mu and the mobile genetic elements Tn3 and IS101 replicatively transpose to random target sites, produce 5 bp target site duplications, and contain the sequence 5'-PuCGAAAPu-3' starting at bp 21 from their ends. The presence of these shared characteristics, plus the fact that Mu transposase can specifically bind to the termini of Tn3 and IS101 in vitro, suggests that the elements may be evolutionarily conserved and retain some functional capacity to transpose each other's DNA. To examine this proposition, in vivo transposition-mating assays were performed and demonstrated that Mu transposase stimulated the formation of recA-independent recombination products between Tn3kan- or IS101-containing plasmids and a target plasmid (pOX38cam) up to 200-fold. However, when transferred to recA+ hosts, these recA-independent products yielded resolution products suggestive of illegitimate recombination, as similar recombination and resolution products were generated, at reduced frequencies, in the absence of Mu transposase. Thus, Mu transposase may stimulate a host-mediated, recA-independent illegitimate recombination reaction. As adjacent pSC101 sequences, including a formerly unknown but functional IHF site (bp 2238-2251), were required for Mu transposase-stimulated IS101 illegitimate recombination, IHF may be one of the putative host factors involved in these recombination reactions.

The bacteriophage Mu transposase protein can form high-affinity protein-DNA complexes with the ends of transposable elements of the Tn 3 family

The 37 kb transposable bacteriophage Mu genome encodes a transposase protein which can recognize and bind to a consensus sequence repeated three times at each extremity of its genome. A subset of this consensus sequence (5'-PuCGAAA(A)-3') is found in the ends of many class II prokaryotic transposable elements. These elements, like phage Mu, cause 5 bp duplications at the site of element insertion, and transpose by a cointegrate mechanism. Using the band retardation assay, we have found that crude protein extracts containing overexpressed Mu transposase can form high-affinity protein-DNA complexes with Mu att R and the ends of the class II elements Tn 3 (right) and IS101. No significant protein-DNA complex formation was observed with DNA fragments containing the right end of the element IS102, or a non-specific pBR322 fragment of similar size. These results suggest that the Mu transposase protein can specifically recognize the ends of other class II transposable elements and that these elements may be evolutionarily related.

Novel rearrangements of IS30 carrying plasmids leading to the reactivation of gene expression

Unusual DNA rearrangements involving the prokaryote mobile genetic element IS30 have been identified. In order to study the potential mechanisms for the reactivation of a gene after IS element insertion, IS30 was introduced between the lacUV5 promoter and the galK gene of the multicopy plasmid pFR100. In this plasmid terminators of RNA transcription in the sequence of IS30 prevent expression of the galK gene from the lacUV5 promoter. A number of independently isolated mutant plasmids re-expressing the galK gene were studied and shown to be tandemly repeated dimers of the original plasmid. However, the two copies of IS30 were tandemly repeated at one of the original sites of insertion, while at the other site IS30 and three base pairs were missing. The repeated copies of IS30 were separated by two base pairs, the same as those originally flanking the elements. An apparently identical mechanism generated cointegrates between a derivative of plasmid pFD51 carrying IS30 upstream of a promoterless galK gene and a derivative of plasmid pACYC177 carrying IS30 inserted into the beta-lactamase gene. This arrangement brought the galK gene under the control of the beta-lactamase promoter of pACYC177. A mechanism involving aborted conservative transposition of IS30 is discussed as a possible route for the generation of these novel cointegrates. In a third experiment we isolated an insert of IS30 which was also two base pairs away from an already resident IS30 element. This insertion of IS30 created a strong promoter of RNA transcription, which has the potential to increase the expression of the transposase in the downstream copy of the element.