Formation of alternative structures by a DNA fragment containing a poly (CA) · poly (TG) tract. a. A 120 bp DNA fragment containing a 60 bp tract of poly (CA) · poly (TG) was 32P end labelled and analyzed on a 4% polyacrylamide gel (lane 1). Under specific conditions of incubation [5,6,7,8] this fragment can give rise to a series of bands (lane 2). Two bands, labelled CA and TG, correspond to the single strands of the fragment. The upper ladder of bands (empty arrowheads) corresponds to multistranded forms . Bands labelled X have not been studied previously and are the subject of the present paper, b. Form X is stable and can be electroeluted (lane 1, see Methods). After elution, Form X was incubated in 10 mM Tris-HCl, 1 mM EDTA, pH 7.5, for 10 min. at the indicated temperatures and analyzed on a polyacrylamide gel, showing that it dissociates at ~ 50-60°C to reform the regular double-stranded fragment, c. Specific interaction of proteins HMG1 and HMG2 with Form X. The starting DNA material (lane 2) contains, in addition to the regular double-stranded fragment, small amounts of single strands and of Form X. In the presence of E. coli competitor DNA, purified HMG1 and HMG2 proteins  (lanes 1 and 3 respectively) bind exclusively to Form X. Increasing the amount of competitor DNA up to 4 μg per sample does not modify the result (not shown). d. Formation of Form X by strand reassociation in the presence of HMG1/2. The DNA fragment, labelled on its TG strand, was heat-denatured and allowed to reassociate in the presence of protein HMG1. By electrophoresis on a polyacrylamide gel, complexes between Form X and HMG1 are obtained (lane 1), and can be dissociated by SDS to yield free Form X (lane 2).