Developmental Control of Gene Expression 
pre-B and surface Ig-positive B cells but not in 
later-stage B cells or T cells. In our study of the 
mb- 1 promoter, we identified a novel nuclear 
factor, termed early B cell factor (EBF) . 
EBF was shown to bind a functionally impor- 
tant sequence in the distal promoter region. The 
factor was found to be present specifically in pre- 
B and surface Ig-positive B cells, thus paralleling 
the pattern of activity of the mb- 1 promoter. Re- 
cently we purified EBF to homogeneity and ob- 
tained partial amino acid sequences. Based on 
this information, we isolated cDNA clones that 
correspond to EBF. Currently we are characteriz- 
ing its DNA-binding and regulatory properties. 
The second approach to identify lineage- 
specific regulators of gene expression consisted 
in differential screening of a pre-B cell (minus 
erythroid) cDNA library. One of the cDNA clones 
was found to encode a DNA-binding protein with 
homology to the chromosomal nonhistone high- 
mobility group protein HMG- 1 and to regulators 
of cell specialization. This pre-B and T cell-spe- 
cific DNA-binding protein, termed LEF-1 (lym- 
phoid enhancer-binding factor 1), was found to 
bind a functionally important site in the T cell 
receptor (TCR) a gene enhancer. 
LEF-1 was shown to participate in the regula- 
tion of the TCRa enhancer. First, co-transfection 
of cells lacking endogenous LEF-1 with TCRa en- 
hancer-containing reporter genes together with 
an LEF-1 cDNA expression plasmid increased en- 
hancer function. Second, in the context of the 
intact minimal TCRa enhancer, the LEF-1 -bind- 
ing site is required for full enhancer function. 
We showed that the HMG domain of LEF- 1 en- 
compasses a sequence-specific DNA-binding do- 
main that interacts with the minor groove of DNA. 
Moreover, we defined amino acids that are con- 
served among various members of the family of 
HMG-domain proteins as residues important for 
DNA binding. 
In previous experiments, we found that LEF-1 
by itself is unable to augment basal promoter ac- 
tivity. This observation raised the possibility that 
LEF-1 functions by aiding the binding and/or ac- 
tion of other nuclear factors. Recently we demon- 
strated that DNA binding by the HMG domain in- 
duces a sharp bend in the DNA helix. Therefore, 
we examined whether LEF- 1 -induced DNA bends 
can facilitate communication between proteins 
bound at distant sites. 
We replaced one binding site for the bacterial 
integration host factor with one for LEF- 1 in the 
att? locus of bacteriophage X and found that 
LEF- 1 is capable of precisely aligning widely sepa- 
rated integrase (Int) protein-binding sites and 
stimulating Int-dependent recombination. These 
data suggest that LEF-1 can serve as an "architec- 
tural" element in the assembly of higher-order 
nucleoprotein structures. Currently we are exam- 
ining the role of DNA bending for the regulation 
of TCRa enhancer function and are attempting 
to gain insight into the importance of LEF- 1 for 
lymphoid differentiation by targeted gene 
inactivation. 
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