Sequence-dependent dynamics of synthetic and endogenous RSSs in V(D)J recombination.

Developing lymphocytes of jawed vertebrates cleave and combine distinct gene segments to assemble antigen-receptor genes. This process called V(D)J recombination that involves the RAG recombinase binding and cutting recombination signal sequences (RSSs) composed of conserved heptamer and nonamer sequences flanking less well-conserved 12- or 23-bp spacers. Little quantitative information is known about the contributions of individual RSS positions over the course of the RAG-RSS interaction.

Evolving adaptive immunity.

Generation of a diverse repertoire of antigen receptor specificities via DNA recombination underpins adaptive immunity. In this issue ofGenes&Development, Carmona and colleagues (pp. 909-917) provide novel insights into the origin and function of recombination-activating gene 1 (RAG1) and RAG2, the lymphocyte-specific components of the recombinase involved in the process.

Single-molecule analysis of RAG-mediated V(D)J DNA cleavage.

The recombination-activating gene products, RAG1 and RAG2, initiate V(D)J recombination during lymphocyte development by cleaving DNA adjacent to conserved recombination signal sequences (RSSs). The reaction involves DNA binding, synapsis, and cleavage at two RSSs located on the same DNA molecule and results in the assembly of antigen receptor genes. We have developed single-molecule assays to examine RSS binding by RAG1/2 and their cofactor high-mobility group-box protein 1 (HMGB1) as they proceed through the steps of this reaction.

Distinct structural features of TFAM drive mitochondrial DNA packaging versus transcriptional activation.

TFAM (transcription factor A, mitochondrial) is a DNA-binding protein that activates transcription at the two major promoters of mitochondrial DNA (mtDNA)--the light strand promoter (LSP) and the heavy strand promoter 1 (HSP1). Equally important, it coats and packages the mitochondrial genome. TFAM has been shown to impose a U-turn on LSP DNA; however, whether this distortion is relevant at other sites is unknown.