Regulation of Gene Expression in Developing Lymphocytes 
identified as the cellular homologue to a retro- 
viral oncogene and was shown only recently to 
bind to specific DNA sequence elements. As with 
most genes expressing DNA-binding proteins, the 
ets- 1 gene is related to several other genes found 
in the mammalian genome, all of which encode 
proteins that bind to similar DNA sequence ele- 
ments. We have expressed in bacteria three dif- 
ferent members of the ets family — ets-1, fli-1, 
and PU.l — and have found that all three bind 
tightly to the TdT D' site. Interestingly, all three 
of these proteins are expressed at high levels in 
certain subsets of hematopoietic cells, and both 
ets-1 and fli-1 are expressed in most, if not all, 
cell types that express the TdT gene. 
We have determined that LyF-1 is not ets-1 or 
fli-1 and is unlikely to be a member of the ets 
family of proteins. Therefore our current goal is 
to determine which of these proteins — LyF-1, 
ets- 1 , fli-1 , or a combination of LyF- 1 and an ets 
member — is required to activate TdT transcrip- 
tion by interacting with the D' element. Those 
proteins required for TdT activation are likely to 
be important for regulating lymphocyte develop- 
ment and will become candidates for involve- 
ment in the deregulation of TdT expression in 
ALL and AML. This work is supported by a grant 
from the National Institutes of Health. 
Unusual Design of the TdT Promoter 
The TdT promoter is unusual in that it does not 
contain a TATA box, which is a common DNA se- 
quence element found in most promoters that 
have been characterized in detail. The TATA ele- 
ment is known to carry out several important 
functions in promoters, including 1) recruitment 
of the RNA polymerase to the transcription start 
site region, 2) determination of the site's loca- 
tion, and 3) determination of the direction of 
transcription from the promoter. To understand 
the regulation of the TdT promoter, we are inquir- 
ing into how these functions are carried out in the 
absence of a TATA box. This question is espe- 
cially important for an understanding of tran- 
scriptional control during lymphocyte develop- 
ment, since it appears that the promoters of most 
genes expressed during this process do not con- 
tain TATA boxes. 
Previously we found that in place of the TATA 
element, which is typically located 30 nucleo- 
tides upstream of the transcription start site, the 
TdT promoter contains a distinct element that in- 
stead overlaps the start site. This element, which 
we call an initiator (Inr) , is like the TATA element 
in its importance for promoter function and also 
in pinpointing the RNA start site to a specific 
nucleotide. 
We are now comparing further the activities 
and mechanisms of action of TATA boxes and Inr 
elements. Currently our data suggest that the 
mechanisms for TATA and Inr-mediated tran- 
scription follow very similar steps prior to the 
actual onset of RNA synthesis. 
We would also like to understand the contribu- 
tions of TATA and Inr elements toward determin- 
ing the start site location and the direction of 
transcription within a promoter. Some promoters 
contain both TATA and Inr elements. Although 
both elements can influence start site localiza- 
tion and transcriptional directionality, the TATA 
element appears to be much more powerful than 
an Inr. Our results, however, also have led us to 
challenge the belief that the orientation of a TATA 
box within a promoter alone determines the di- 
rection of transcription. Instead, our data suggest 
that the direction of transcription within a simple 
promoter is determined by the location of a TATA 
box or Inr element relative to a specific activator 
element. 
We are continuing to define the rules by which 
a promoter determines the direction of RNA syn- 
thesis in order to understand further the design of 
more-complex promoters, including those that 
contain, like the TdT promoter, important se- 
quence elements both upstream and downstream 
of the transcription start site. 
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