Insertion elements

XII. THE GENETICS OF SYMBIOSIS A FAIR OF PLASMIDS AND A NIGHTMARE OF INSERTION ELEMENTS. WHY ... 

The evolutionary history of symbiotic nitrogen fixers is therefore a tale of coevolution, which occurred in the shadow of their hosts, chasing their growing roots, and striving for adaptation. It is an example of how bacterial genetics has managed to keep pace with the creative power of eukaryotic sexual recombination. Mobile replicons, insertion elements, and symbiotic islands prone to move have helped rhizobia to succeed in their pursuit. The race, naturally, is not over and, looking at it from a distance, what we have. seen, compared to what we have yet to see, is probably just a cloud of dust. 

K. Kaluza, M. Hahn, and H. Hennecke, Repeated sequences similar to insertion elements clustered around the nif region of the Rhizobium japonicum genome. J. Bacteriol. 162 535 (1985). 

U. B. Briefer, H. J. Burkhardt, W. Klipp, and A. Piihler, ISRl, an insertion element isolated from the soil bacterium Rhizobium lupini. Cold Spring Harbor Symp. Quant. Biol. 45 87 (1980). 

Fu R, Voordouw G. 1998. ISD7, an insertion element from the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough structure, transposition, and distribution. Appl Environ Microbiol 63 53-61. 

Katinakis, P. Verma, D.P.S. (1985). Nodulin-24 gene of soybean codes for a peptide of the peribacteroid membrane and was generated by tandem duplication of an insertion element. Proceedings of National Academy of Sciences (USA) 82, 4157-61. 

The influence of transposons and insertion elements in basic mutagenesis is just beginning to be understood, and it is not known how pervasive or important transposition is in mammals. This is a very active field, and we expect that important results will be available soon. 

The computer search of the NBRF protein database with the Loa element identified the ribosomal insertion element R1 from D. melanogaster as the best fit. Other significant fits were obtained with the Jockey... 

Shure et al. (1983) prepared cDNA clones homologous to Wx mRNA. In subsequent experiments (Federoff et al., 1983), restriction endonuclease fragments containing part of the Wx locus were cloned from strains carrying the ac wx-M9, wa-M9, and wx-M6 alleles to characterize further the controlling insertion elements activator (ac) and dissociation (ds). Excision of the ds element from the certain wx alleles produces two new alleles (S5 and S9) that are encoding the wx proteins having altered starch synthase activities (Wessler et al., 1986). Two of these, S9 and S5, had 53 and 32% of the starch synthase activity, respectively, seen in the normal endosperm. Mutant... 

Sec) (9). In bacteria, Sec is inserted cotranslationaUy at an in-frame UGA codon that is upstream of an RNA stem-loop. In E. coli, SerRS produces the mischarged Ser-tRNA , which is then converted to Sec-tRNA by Sec synthase (SelA). A GTP-dependent elongation factor SelB binds to Sec-tRNA and forms a complex that recognizes specific mRNA sequences called selenocysteine insertion elements (SECIS), that are located 3 to a UGA codon on a stalled ribosome-bound mRNA. It is also responsible for delivering Sec-tRNA to the A site of the ribosome (9). 

Sequence Insert element Delete element Change element No change... 

Several bacterial transposons, referred to as insertion elements (ISelements), consist only of a gene that codes for a transposition enzyme (i.e., transposase), flanked by short DNA segments called inverted repeats (Figure 18.16). (Inverted repeats are short palindromes.) More complicated bacterial transposable elements, called composite transposons, contain additional genes, several of which may code for antibiotic resistance. Because transposons can jump between bacterial chromosomes, plasmids, and viral genomes, transpositions are now believed to play an important role in the spread of antibiotic resistance among bacteria. 

Fennewald, M., S. Benson, M. Oppici, and J. Shapiro. 1979. Insertion element analysis and mapping of the Pseudomonas plasmid alk regulon.. Bacteriol. 139 940-952. 

Transposable elements are mobile DNA segments that can insert themselves into many sites in cellular DNA (7 ). Transposable elements are not capable of autonomous replication but are propagated only when they are inserted into chromosomal or plasmid DNA. Insertion sequence elements (IS elements) are the smallest pieces of DNA (0.7 to 1.5 kb) known to function as transposable elements. IS elements contain only the genes needed for their insertion into cellular DNA. Transposons (Tn elements) are more complex transposable elements that do contain genes unrelated to the insertion function. Transposons frequently have a central piece of DNA containing one or several genes flanked by nearly identical insertion elements at its termini (Figure 2, 8). 

Figure 2. Generalized structure of a hypothetical catabolic transposon encoding three enzymes (A, B, and C) involved in the degradation of a pesticide. The clear zones marked IS-L and IS-R represent the nearly identical insertion elements that mark the left and right boundries of the transposon and encode the proteins required for movement of the transposon.

The relatively large central region of an IS element, which encodes one or two enzymes required for transposition, is flanked by an inverted repeat at each end. The sequences of the inverted repeats are nearly identical, but they are oriented in opposite directions. The sequence is characteristic of a particular IS element. The 5 and 3 short direct (as opposed to inverted repeats are not transposed with the insertion element rather, they are insertion-site sequences that become duplicated, with one copy at each end, during insertion of a mobile element. 

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