A functional component of the sea urchin H2A gene modulator contains an extended sequence homology to a viral enhancer.

The DNA sequences imparting a maximal rate of sea urchin H2A gene transcription in the frog oocyte nucleus were narrowed down by deletion mapping to a DNA segment -165 to -111, far-upstream of the H2A mRNA cap site. C to T base changes in this area create strong down mutations, hence the primary structure of this DNA sequence is of paramount importance to the H2A gene expression. Sequence comparisons suggest that the -165 to -111 region may contain two essential sequence blocks. Most strikingly, the -135 area contains a 14 out of 17 basepair homology to the Moloney murine sarcoma virus enhancer and to topologically related 5' LTR-sequences of the simian sarcoma virus and the murine Friend spleen focus forming virus.