Mechanism of Mu DNA transposition

Retroviral integrase: Structure, mechanism, and inhibition

The retroviral protein Integrase (IN) catalyzes concerted integration of viral DNA into host chromatin to establish a permanent infection in the target cell. We learned a great deal about the mechanism of catalytic integration through structure/function studies over the previous four decades of IN research. As one of three essential retroviral enzymes, IN has also been targeted by antiretroviral drugs to treat HIV-infected individuals. Inhibitors blocking the catalytic integration reaction are now state-of-the-art drugs within the antiretroviral therapy toolkit. HIV-1 IN also performs intriguing non-catalytic functions that are relevant to the late stages of the viral replication cycle, yet this aspect remains poorly understood. There are also novel allosteric inhibitors targeting non-enzymatic functions of IN that induce a block in the late stages of the viral replication cycle. In this chapter, we will discuss the function, structure, and inhibition of retroviral IN proteins, highlighting remaining challenges and outstanding questions.

Tn552, a novel transposable element from Staphylococcus aureus

Tn552, one of several closely related beta-lactamase-encoding transposons from Staphylococcus aureus, has a novel set of putative transposition functions. Each is homologous with a well-characterized function from a different type of mobile genetic element. Thus, Tn552 encodes: (i) resL-binL, a co-integrate resolution system homologous with those of Tn3 family elements; (ii) p480, a potential transposase significantly homologous with the DNA integrases of eukaryotic retroviruses and retrotransposons; and (iii) p271, a potential ATP-binding protein that shows homology with the B protein of phage Mu. The 3' terminal nucleotides of Tn552 (CA), adjacent to which p480 might cleave, are the same as those of retroviruses, retrotransposons and phage Mu. The presumptive resolvase (BinL) is very closely related to BinR, which was identified as a DNA invertase and is now shown to resolve an artificial co-integrate in vivo. Furthermore, the structure of the derivative of Tn552 found in the staphylococcal plasmid pI258 can be explained by a BinL (or BinR)-mediated site-specific deletion ('resolution') event. Thus, pI258 contains only the right-hand half of Tn552, which encodes the beta-lactamase and two regulatory proteins. The latter are homologous with the beta-lactamase gene repressor and co-inducer of Bacillus licheniformis. Interestingly, the order of the regulatory genes is reversed in S. aureus compared with Bacillus licheniformis.

Secondary structural features of the bacteriophage Mu-encoded A and B transposition proteins

The role of the bacteriophage Mu-encoded A and B proteins is to direct the transposition of Mu DNA. These are the first active DNA transposition proteins to have been purified and their mechanism of action at the biochemical level is under intensive study. Structural studies on these proteins, however, have lagged behind their biochemical characterization. We report here near- and far-u.v. c.d. spectra for these proteins and their secondary structural features derived from these data. The Mu A protein appears to be composed of primarily beta-sheet (40%) with 24% alpha-helix, 9% beta-turn and 27% random coil. In contrast, the Mu B protein contains 55% alpha-helix with only 13% beta-sheet and 3+ beta-turn and 29% random coil. The near-u.v. c.d. spectrum of the A protein was not unusual; however, the profile of the B protein suggested either buried or restricted chromophores within the protein or short-range interactions between aromatic residues.