The binding of a transcription factor to deletion mutants of a 5S ribosomal RNA gene. 
A transcription factor binds specifically to a region within the 5S ribosomal RNA gene that is known to be required for accurate initiation of transcription of the gene.  We have confirmed the significance of this interaction by comparing various deletion mutants for their ability to initiate transcription and to bind the protein.  Loss of specific binding of the protein occurs when the gene is deleted beyond +83 from the 3' direction.  This coincides exactly with the loss of ability to initiate transcription.  In contrast, deletions entering the gene from the 5' direction and extending beyond the 5' border of the control region (+50/+55) give partial protection of the remainder of the control region.  This observation can explain why deletions beyond the 5' border of the control region that are unable to initiate transcription can still compete for the transcription of the wild-type 5S RNA gene.