We have inserted the 509-bp-long fragment of sea urchin P. miliaris histone gene spacer region into plasmid pUC19. The fragment contains the 60-bp-long homopurine-homopyrimidine tract that is known to be hypersensitive to the S1 endonuclease. Using two-dimensional gel electrophoresis we have observed a sharp structural transition in the insert with increasing DNA superhelicity. As in the cases of cruciform and Z form formation, the observed transition partly relaxes the superhelical stress. In contrast with the other two well documented transitions, the observed transition strongly depends on pH. At pH7 and above the transition occurs at negative superhelicities exceeding the physiological range (- sigma greater than 0.08). For pH6 the transition occurs at -sigma = 0.055, whereas for pH4.3 it takes place at -sigma = 0.001. A comprehensive analysis of the obtained data has made it possible to define the nature of the observed transition. We conclude that under superhelical stress or/and at low pH homopurinehomopyrimidine tracts adopt a novel spatial structure called the H form.