M. L.R. Smith

The influence of transcriptional orientation on endogenous switch region function

Immunoglobulin heavy chain (IgH) class switch recombination (CSR) takes place between large switch (S) regions that precede exons of the constant region. The precise functions of the S region are controversial, although transcription of the S region targets CSR. We have tested the effects of deletion, inversion and replacement of the endogenous 12-kilobase Sγ1 region on CSR in vivo. Here we show that Sγ1 is required for CSR, that CSR is effected by a 1-kilobase sequence that generates a G-rich transcript, and that inversion of Sγ1 or the G-rich sequence decreases CSR. We conclude that Sγ1 function is dependent on orientation and lacks an absolute requirement for common S region motifs. We propose that single-stranded DNA stabilized by transcription-dependent, higher order structures is a primary substrate of CSR.

Deficiency in the Nuclease Activity of Xeroderma Pigmentosum G in Mice Leads to Hypersensitivity to UV Irradiation

ABSTRACT Xeroderma pigmentosum (XP) is a human disorder which is characterized by hypersensitivity to sunlight and elevated incidence of skin cancer. The disease is caused by mutations in genes that encode components of the nucleotide excision repair pathway. The gene product of XP complementation group G (XPG) is a structure-specific endonuclease which makes an incision 3′ to DNA photoproducts and other helix-distorting DNA adducts. In addition, the XPG protein has been implicated in transcription and repair of oxidative DNA damage. Moreover, XPG is capable of cleaving R loops in vitro, a potential intermediate during immunoglobulin heavy-chain class switch recombination. Due to its multiple functions, complete elimination of XPG in mice results in severe postnatal growth defects and premature death. To understand the contribution of the XPG nuclease activity to its function in vivo, we introduced a point mutation into the mouse XPG gene which inactivates the nuclease catalytic site but leaves the remainder of the protein intact. The XPG nuclease-deficient animals develop normally and exhibit no obvious defect in class switch recombination. However, the mutant mice are hypersensitive to UV irradiation. This phenotype suggests that the nuclease activity of XPG is required only for nucleotide excision repair and that other regions of the protein perform independent functions.

Minting new COIN: critiquing counter-insurgency theory

Over the last half-decade, counter-insurgency (COIN) rose to prominence as the dominant paradigm in American and British thinking about the wars in Iraq and Afghanistan, and indeed for the presumed wars of the future. ‘COIN’ achieved such currency in the strategic community that it became more than a military doctrine, which is its nominal status. Instead, it became a universal panacea. It offered a strategy, a theory of warfare, a movement in defence and military circles, and a ‘how to’ guide for implementing an interventionist American and allied foreign policy, informed by a seemingly humanitarian orientation.1