In T-cell precursors, the T-cell-receptor beta chain is expressed before the T-cell-receptor alpha chain and is sufficient to advance T-cell development in the absence of T-cell receptor alpha chains. In immature T cells, the T-cell-receptor beta protein can form disulphide-linked heterodimers with the pre-T-cell-receptor alpha chain and associate with signal-transducing CD3 molecules. The recently cloned pre-T-cell-receptor alpha gene encodes a transmembrane protein that is expressed in immature but not mature T cells. Here we show that alpha beta, but not gamma delta, cell development is severely hampered in pre-T-cell-receptor alpha-gene-deficient mice, which establishes a crucial role for the pre-T-cell receptor in early thymocyte development.

Mechanisms of DNA repair and mutagenesis are defined on the basis of relatively few proteins acting on DNA, yet the identities and functions of all proteins required are unknown. Here, we identify the network that underlies mutagenic repair of DNA breaks in stressed Escherichia coli and define functions for much of it. Using a comprehensive screen, we identified a network of ≥93 genes that function in mutation. Most operate upstream of activation of three required stress responses (RpoS, RpoE, and SOS, key network hubs), apparently sensing stress. The results reveal how a network integrates mutagenic repair into the biology of the cell, show specific pathways of environmental sensing, demonstrate the centrality of stress responses, and imply that these responses are attractive as potential drug targets for blocking the evolution of pathogens.

The T cell antigen receptor (TCR) beta chain regulates early T cell development in the absence of the TCR alpha chain. The developmentally controlled gene described here encodes the pre-TCR alpha (pT alpha) chain, which covalently associates with TCR beta and with the CD3 proteins forms a pre-TCR complex that transduces signals in immature thymocytes. Unlike the lambda 5 pre-B cell receptor protein, the pT alpha chain is a type I transmembrane protein whose cytoplasmic tail contains two potential phosphorylation sites and a Src homology 3 (SH3)-domain binding sequence. Pre-TCR alpha transfection experiments indicated that surface expression of the pre-TCR is controlled by additional developmentally regulated proteins. Identification of the pT alpha gene represents an essential step in the structure-function analysis of the pre-TCR complex.