Calf thymus histone H1 is a recombinase that catalyzes ATP-independent DNA strand transfer.

An activity that catalyzes the strand transfer from linear double-stranded tetracycline-resistance gene (tetr) DNA to circular M13mp8-tetr viral DNA was detected in a crude extract from calf thymus. This activity was purified to near, if not complete, homogeneity as judged by NaDodSO4/polyacrylamide gel electrophoresis. We have tentatively named this protein calf thymus strand-transfer protein 1 (CTST1). The apparent molecular mass of the protein was 35 kDa by gel electrophoresis. Its sedimentation coefficient was approximately 1.5 S in glycerol gradient centrifugation. These values led us to examine the possibility that CTST1 is histone H1. Western blot analysis of CTST1 with anti-rat liver histone H1 antiserum showed that CTST1 crossreacts with the serum, indicating that CTST1 is histone H1. The mobility of CTST1 was identical to one of the subtypes of calf thymus histone H1 by NaDodSO4/polyacrylamide gel and acetic acid/urea/polyacrylamide gel electrophoreses. We have also confirmed the above conclusion by showing that calf thymus histone H1 has a strand-transfer activity with a specific activity comparable to that of CTST1. The reaction required homologous substrates, but neither Mg2+ nor ATP. The reaction also required stoichiometric amounts of protein. The purified CTST1 fraction lacked detectable exo- and endonuclease activities and also lacked a DNA helicase activity.