In vivo footprints are found in the Xenopus 63 kDa keratin gene promoter prior to the appearance of mRNA.

Previous work on the promoter region of the 63 kDa keratin gene demonstrated that in vivo footprints did not change during the transition from low-level to high-level transcription. Reverse transcription polymerase chain reaction and in vivo footprinting were used to determine if these DNA-protein interactions are present before transcription begins. The results presented indicate that during development, DNA-protein interactions are present in the promoter region of the 63 kDa keratin gene at stage 44, four days prior to the initial appearance of 63 kDa keratin mRNA, at stage 48. Thus, the occupancy of these sites at stage 44 is not sufficient for transcription, but may have a role in 'poising' the keratin promoter for the initiation of epidermal-specific transcription. The results suggest that the developmental history of a gene may be important in regulating its temporal and spatial expression.