Chromosomal proteins HMG-14 and HMG-17 enhance the transcriptional potential of chromatin when incorporated into nucleosomes during, but not after, chromatin assembly on replicating DNA. Two molecules of either HMG-14 or HMG-17 can bind to nucleosome cores, independently of the underlying DNA sequence, in a cooperative fashion to limit nucleosome mobility and stabilize the structure of the nucleosome core without stabilizing the higher order chromatin structure. By modifying the structure of nucleosomes, the proteins affect the local structure of the chromatin fiber leading to an increase in the rate of transcriptional elongation but not initiation. We suggest that HMG-14/-17 are architectural elements which assist in the assembly of an unfolded chromatin fiber thereby decreasing the repressive activity of histones and facilitating transcriptional processes.