Regulation of Gene Expression in Developing Lymphocytes 
Many other genes expressed in early lymphocytes 
are also expressed in other tissues. Second, un- 
like the Ig genes, which are permanently acti- 
vated during B cell differentiation, the TdT gene 
is only transiently activated and then remains off 
in the mature cell. This expression pattern pro- 
vides the opportunity to analyze mechanisms for 
transcriptional activation as well as inactivation 
during lymphopoiesis. Third, the TdT gene is not 
only expressed in early B cells, but also in devel- 
oping T lymphocytes, which are responsible for 
the cell-mediated immune response. It will 
therefore be interesting to analyze common regu- 
latory proteins and pathways for these two dis- 
tinct lymphoid lineages. And finally, until the re- 
cent isolation of two new genes, called RAG-1 
and RAG-2, TdT was the only gene available 
whose product was thought to play a specific role 
in the gene rearrangement process responsible 
for antibody diversity. Because the rearrange- 
ments are likely to be key events that drive B and 
T lymphocyte differentiation, it is important to 
investigate the regulation of the rearrangement 
machinery in order to understand the control of 
lymphopoiesis. 
Our analysis of TdT regulation has suggested, 
quite surprisingly, that the TdT control mecha- 
nisms are fundamentally very different from Ig 
control mechanisms. Moreover, the TdT tran- 
scriptional control mechanisms may be represen- 
tative of those for a variety of genes expressed in 
early B and/or T cells. 
Both the general structure of the transcrip- 
tional control region within the TdT locus 
(called the promoter) and the lymphocyte-spe- 
cific DNA-binding proteins for TdT transcription 
appear to be unique when compared with our 
knowledge of Ig transcription. The TdT promoter 
does not contain a TATA box, a common DNA se- 
quence element found in the Ig promoters and in 
most other promoters that have been character- 
ized in detail. Moreover, the lymphocyte-specific- 
ity of the TdT promoter appears to be regulated in 
part by a novel lymphocyte-specific DNA-binding 
protein, called LyF- 1 , rather than by the OCT-2 or 
NF-kB proteins. 
Most importantly, these features of the TdT 
promoter are found in the promoters for a num- 
ber of other genes activated in early B and/or T 
cells. These include the B cell-specific X5 and 
VpreB genes and the two distinct promoters for 
the T cell-specific Ick gene. All of these pro- 
moters lack TATA boxes and contain at least two 
binding sites for LyF- 1 . 
Our studies currently are focused on under- 
standing the two unique qualities of TdT tran- 
scription described above: 1) the ability to direct 
accurate RNA synthesis in the absence of a TATA 
element and 2) the apparent use of the LyF-1 pro- 
tein to control the lymphocyte specificity of TdT 
expression. 
In place of the TATA element, which typically 
is located 30 nucleotides upstream of the tran- 
scription start site, the TdT promoter contains a 
distinct element that overlaps the start site. This 
element, which we call an initiator (Inr), is like 
the TATA element in that it is important for pro- 
moter function and also pinpoints the RNA start 
site to a specific nucleotide. We recently have 
found functional Inr elements in many different 
genes, even though a precise, consensus DNA se- 
quence has not been detected. 
We have also found, through analysis of syn- 
thetic promoters, that the TATA element is domi- 
nant over the Inr element. In a promoter contain- 
ing both TATA and Inr, the TATA element directs 
the location of the transcription start sites. Other 
studies are providing further insights into the 
functioning of the Inr element and its relation- 
ship to TATA. 
The LyF-1 protein appears to play a role in the 
lymphocyte specificity of TdT transcription. It is 
found predominantly in lymphoid cells and binds 
tightly to two sites in the TdT promoter that are 
important for lymphocyte-specific transcription. 
We have purified this protein in order to demon- 
strate that it can bind to promoters for the other 
lymphocyte-specific genes mentioned above. 
Moreover, the purified protein is being used to 
reach one of our primary goals: isolation of the 
gene encoding LyF- 1 . This gene will allow us to 
understand better the mechanism by which LyF- 1 
regulates TdT expression. Ultimately, our contin- 
ued analyses of LyF- 1 and of TdT expression are 
likely to broaden our knowledge of the control of 
hematopoiesis. 
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