ONE EXON OF THE HUMAN LSF GENE INCLUDES CONSERVED REGIONS INVOLVED INNOVEL DNA-BINDING AND DIMERIZATION MOTIFS

The transcription factor LSF, identified as a HeLa protein that binds the simian virus 40 late promoter, recognizes direct repeats with a ce nter-to-center spacing of 10 bp. The characterization of two human cDN As, representing alternatively spliced mRNAs, provides insight into th e unusual DNA-binding and oligomerization properties of LSF. The seque nce of the full-length LSF is identical to that of the transcription f actors alpha CP2 and LBP-1c and has similarity to the Drosophila trans cription Factor Elf-1/NTF-1. Using an epitope-counting method, we show that LSF binds DNA as a homodimer. LSF-ID, which is identical to LBP- 1d, contains an in-frame internal deletion of 51 amino acids resulting from alternative mRNA splicing. Unlike LSF, LSF-ID did not bind LSF D NA-binding sites. Furthermore, LSF-ID did not affect the binding of LS F to DNA, suggesting that the two proteins do not interact. Of three s hort regions with a high degree of homology between LSF and Elf-1/NTF- 1, LSF-ID lacks two, which are predicted to form P-strands. Double ami no acid substitutions in each of these regions eliminated specific DNA -binding activity, similarly to the LSF-ID deletion. The dimerization potential of these mutants was measured both by the ability to inhibit the binding of LSF to DNA and by direct protein-protein interaction s tudies. Mutations in one homology region, but not the other, functiona lly eliminated dimerization.