ever, they also found that RAG-1, but apparently not 
RAG- 2, is transcribed in neurons in the central ner- 
vous system, raising the possibility that RAG-1 has a 
role in processes other than classic V(D)J recombi- 
nation. Dr. Schatz's laboratory is currently testing 
the hypothesis that RAG- 1 is involved in recombina- 
tion in the central nervous system by studying the 
behavior of recombination substrates in neuronal 
cell lines and by characterizing circular DNA mole- 
cules (thought to be the by-product of recombina- 
tion) isolated from the central nervous system. 
Because of their intimate connection with V(D)J 
recombination, RAG-1 and RAG-2 provide impor- 
tant tools for the study of the molecular mechanisms 
regulating the recombination process. Using them 
as indicators, Dr. Schatz and his colleagues are study- 
ing when and how the V(D)J recombinase is turned 
on and then off again as B and T cells develop. Initial 
studies, done in collaboration with Dr. Craig B. 
Thompson (HHMI, University of Michigan) and his 
colleagues, demonstrated that signals transduced 
through surface T cell receptor molecules are capa- 
ble of down-regulating expression of RAG-1 and 
RAG-2 in thymocytes, a result that has helped estab- 
lish a new paradigm for how expression of the V(D)J 
recombinase is terminated. 
V(D)J recombination is regulated not only at the 
level of expression of the recombinase, but also at 
the level of the availability, or "accessibility," of 
the individual immunoglobulin and T cell receptor 
gene segments. Dr. Schatz and his colleagues have 
demonstrated that nonlymphoid cells expressing 
high levels of RAG-1 and RAG-2 do not recombine 
their endogenous immunoglobulin or T cell recep- 
tor gene segments, despite their high levels of re- 
combinase activity. The laboratory is currently de- 
termining whether the introduction of cloned 
transcription factors, known to bind to the en- 
hancers and promoters of immunoglobulin and T 
cell receptor genes, into such nonlymphoid cells 
can activate the rearrangement of immunoglobulin 
and T cell receptor gene segments. Such reconstruc- 
tion experiments could provide a more detailed un- 
derstanding of how DNA structure affects the V(D)J 
recombination process. 
Finally, a central goal of Dr. Schatz's laboratory is 
to understand the enzymatic mechanism of V(D)J 
recombination, an elusive goal thus far, largely be- 
cause efforts to reconstitute the reaction in a cell- 
free system have been unsuccessful. As a first step 
toward this goal, the group is working to obtain 
highly purified preparations of the RAG-1 and 
RAG-2 proteins, and antibodies that specifically rec- 
ognize these proteins. With these tools, the labora- 
tory will examine the biochemical properties of the 
RAG proteins, asking in particular whether they ex- 
hibit the activities expected of proteins involved in 
recombination (e.g., topoisomerase, endonuclease, 
exonuclease, or ligase activities) and whether they 
bind DNA in a sequence-specific manner. Also to be 
addressed are questions concerning the cellular and 
subcellular localization of the RAG proteins and the 
identity of other proteins, if any, with which they 
interact. (This aspect of the laboratory's work was 
supported by a grant from the National Institutes of 
Health.) 
Dr. Schatz is also Assistant Professor of Immu- 
nobiology at Yale University School of Medicine. 
Articles 
Schatz, D.G., and Chun, J.J. M. 1992. V(D)J recom- 
bination and the transgenic brain blues. New Biol 
4:188-196. 
Schatz, D.G., Oettinger, M.A., and Schlissel, M.S. 
1992. V(D)J recombination: molecular biology 
and regulation. Annu Rev Immunol 10:359- 
383. 
REGULATED AND BASAL TRANSCRIPTION OF LYMPHOCYTE-SPECIFIC GENES 
Stephen T. Smale, Ph.D., Assistant Investigator 
Mature B and T lymphocytes arise from hemato- 
poietic stem cells through a complex but highly or- 
dered sequence of events. A central component of 
this developmental process is the precise activation 
and inactivation of a large pool of genes expressed 
with a wide variety of temporal and cell-specific 
patterns. 
Dr. Smale's laboratory examines the mechanisms 
responsible for gene regulation in developing lym- 
phocytes. These studies focus on two distinct areas. 
First, an analysis of the terminal deoxynucleotidyl- 
transferase (TdT) gene is directed toward identify- 
ing transcription factors responsible for restricting 
gene expression to lymphoid cells. Second, studies 
of core promoter regions found in lymphocyte- 
specific genes are conducted to elucidate general 
IMMUNOLOGY 357 
