T lymphocyte ontogeny is marked by a series of changes in the expression of certain glycoproteins on the cell surface. Two of these proteins, designated T4 and T8, are structurally and evolutionarily related to the immunoglobulins. Activation of the T4 and T8 genes occurs during early T-cell development, and the vast majority of immature thymic lymphocytes express both molecules simultaneously. Mature T-cells, however, invariably express only one or the other of these proteins: helper T-cells generally bear surface T4, whereas most suppressor/cytotoxic T-cells express T8. Our goal is to identify the genetic regulatory elements that control T4 and T8 transcription, with particular emphasis upon the molecular mechanisms which produce their selective expression in different T-cell subsets. This knowledge will be of value not only in understanding how the activity of specific eukaryotic genes is regulated, but also in elucidating at a molecuar level the process by which T lymphocytes differentiate and acquire their normal functions in immunity. Using techniques of recombinant DNA, we propose to analyze the structure and transcriptional function of the human T4 and T8 genes, both in their native chromatin environments and after transfection into lymphoid and non-lymphoid recipient cells. Selected portions of these genes will be systematically deleted, modified, or linked to heterologous promoters in order to delineate the precise sequence elements that contribute to transcriptional regulation. We will investigate the mechanisms of action of these regulatory elements, as well as their interactions with each other and with the intracellular milieu. By comparing the regulatory elements of T4 and T8 to those controlling immunoglobulin expression, we hope to learn how these two related gene systems have diverged in evolution to become uniquely expressed by the T- and B-cell lineages, respectively. Finally, we will use the knowledge gained in these experiments to extend our analysis of gene regulation to an even more fundamental level: the isolation and characterization of trans-acting factors that control T lymphocyte gene expression in a tissue-specific or subset-specific fashion.