Chapter 30 : Tn7

Tn7 encodes two pathways that recognize different types of target site. One pathway is directed by the TnsABC+TnsD proteins and directs insertions into a single site, called attTn7, found in bacteria. Insertion into the attTn7 site does not appear to harm the bacterial host and likely maximizes vertical transmission of the element. A second pathway directed by the TnsABC+TnsE proteins directs insertions into plasmids capable of mobilizing between bacteria when they enter the host. Insertion into mobile plasmids probably facilitates the horizontal transfer of the element. Neither pathway is likely to inactivate host genes. The positions of the five Tn7 genes required for transposition, tnsA, tnsB, tnsC, tnsD, and tnsE, are shown including the “Variable region” that contains genes likely to benefit the element and/or the bacterial host. The right and left ends are indicated with black triangles and showing the layout of the seven 22-bp TnsB binding sites with black arrows.

Mechanisms of DNA transposition. Canonical cut-and-paste transposition involves two separate reactions (left side). Canonical replicative transposition involves a direct joining reaction to the target DNA and extensive DNA replication, often along with resolution of a co-integrate (right side). Tn7 utilizes a heteromeric transposase that is capable of directly joining the broken ends of the element to a target DNA, but does not require extensive processing after transposition because it has a second protein, the TnsA endonuclease (center). The rectangle indicates the transposon element and the triangles indicate the cis-acting sequences at the ends of the element. (See the text for details.)

The relationship between the positions of Tns-protein and host-protein binding relative to the point of insertion and the orientation of the element. In TnsABC+TnsD-mediated transposition into the attTn7 site, TnsD binds to the very C-terminal coding region of the glmS gene (not shown). Two host proteins, ACP and L29, help in this binding reaction (cross-hatched ovals). Approximately 25 TnsA2TnsB2 complexes are recruited to the attTn7 site by TnsD, which appear to encompass a region across both sides of the point of insertion. Based on the findings with TnsD and the TnsABC+TnsE in vitro reaction, a plausible model for TnsE-mediated targeting of the lagging-strand template during DNA replication has the protein to the left of the point of insertion and the orientation of the element. TnsE interacts with the 3′ recessed end structure and the sliding clamp processivity factor protein (cross-hatched ring). TnsE may recruit a similar array of TnsA2TnsB2 complexes as found with the TnsABC+TnsD complexes.

The relevant features of the Tns proteins of Tn7 and their total length. TnsA encodes a conserved N-terminal catalytic domain [77 to 168 amino acids (aa)] as indicated and the amino acids that coordinate the metal are shown (E63, D114, and K132). The conserved C-terminal region of TnsA is shown (170 to 248 aa) as is the region that interacts with TnsB, TnsC, and DNA. TnsB encodes a conserved catalytic domain (266 to 406 aa) and the amino acids that coordinate the metal are shown (D273, D361, and E396). The regions of TnsB that interact with TnsA (440 to 480 aa) and TnsC (662 to 702 aa) are indicated. TnsC contains a conserved domain from the AAA family (126 to 281 aa). The 1 to 293 region of TnsC interacts with TnsD and the 504 to 555 region of TnsC interacts with TnsA. TnsD has a CCCH zinc finger motif (C124, C127, C152, H155) and the region 1 to 309 interacts with TnsC. TnsE contains a sliding clamp interacting motif 121 to 131 aa and may generally act with DnaN across the N-terminus of the protein and DNA with the C-terminus of the protein based on TnsE gain-of-activity and loss-of-activity mutations. (See the text for details.)

The TnsABC+D pathway directs transposition between glmS and a downstream gene. The downstream genes will differ by the particular genus and species (shown as orfX and orfY) (A). The TnsD protein recognizes sequences in glmS and directs insertions in one orientation at a single position downstream of glmS and this insertion will have the 5-bp target-site duplication (TSD) associated with this element (B). Additional Tn7-like elements that have diverged from the others can insert in series and can accumulate with new elements always inserting proximal to glmS at the exact same position and with the same target-site duplication (C). Over time the elements will pick up inactivating mutations and deletions that will contribute to the element eroding leading to a mix of functional and nonfunctional elements (D). Genomic islands can result when the transposase genes and ends are lost, but highly selected genes still reside in the attTn7 site (E). (See the text for details.)

Target immunity inhibits addition copies of the element from occurring in the same site or region where one already exists. The target immunity process is mediated by the Tn7 proteins TnsA, TnsB, and TnsC. TnsB that is bound to the ends of the element, and presumably therefore at a higher concentration in this region, will not allow TnsC to form a productive target complex with TnsD and TnsE. This behavior is modeled to redistribute active TnsC to other sites where stable complexes can form with TnsA and the target-site selection proteins. (See the text for details.)

Comparison of various transposable elements. Representative transposons that utilize a transposase regulator protein (TnsC/TniB). The Tn5090/Tn5350 element undergoes replicative transposition where a resolvase (TniR) is used to resolve co-integrates at the res site after transposition and replication of the element. Tn5090/Tn5350 is compared with the known or putative heteromeric transposase elements Tn7, Tn6022, and Tn6230. (See the text for details.)

Phylogenetic tree of TnsA endonuclease transposase found with putative heteromeric transposase elements indicated by their host with Tn7, Tn6230, and a Tn6022 derivative (Tn6022) also indicated. A MUSCLE alignment file of extracted TnsA N-domain sequences (Tn7_Tnp_TnsA_N) (PF08722) with eight iterations was used to build the tree using the Jukes-Cantor genetic distance model and a Neighbor-Joining tree-building method ( 112 , 113 , 114 ) using the following sequences: Acidithiobacillus ferrooxidans ATCC 33020 (AAC21667) ( 115 ), A. ferrooxidans ATCC 53993 (YP_002220549), Acidovorax sp. JS42 (WP_011804647), Ac. baumannii AYE (WP_012300781) ( 116 ), Aeromonas salmonicida subsp. salmonicida A449 (WP_005317426) ( 117 ), Bacillus cereus ATCC10987 (WP_001129185) ( 118 ), B. thuringiensis DAR 81934 pNB4711 (WP_017762552) ( 119 ), Desulfovibrio oxyclinae DSM 11498 (WP_018123630), Tn7 E. coli ( 120 ), Glaciecola polaris LMG 21857 (WP_007103512), Glaciecola sp. HTCC2999 (WP_010179396), Hahella ganghwensis DSM 17046 (WP_020406004), Idiomarina loihiensis L2TR (WP_011235836) ( 121 ), Neisseria wadsworthii 9715 (WP_009116837), Pelobacter carbinolicus DSM 2380 (WP_011339779) ( 122 ), Pseudomonas syringae pv. tabaci str. 6605 (WP_016981931), P. syringae pv. Maculicola str. ES4326 (WP_007247747), Rheinheimera sp. A13L (WP_008897114) ( 123 ), Saccharophagus degradans 2-40 (WP_011466674) ( 124 ), S. enterica subsp. enterica serovar Senftenberg str. A4-543 (EHC89275) ( 125 ), Shewanella baltica OS155 (WP_011848289) ( 126 ), and Wohlfahrtiimonas chitiniclastica SH04 (WP_008315655). (See the text for details.)