DNA strand transfer protein beta from yeast mitotic cells differs from strand transfer protein alpha from meiotic cells.

We have purified to homogeneity an activity from mitotic cell extracts of the yeast Saccharomyces cerevisiae, which promotes the transfer of a strand from a duplex linear DNA molecule to a complementary circular single strand. This activity does not require any nucleotide cofactor and is greatly stimulated by yeast single-stranded DNA-binding protein. It consists of a single polypeptide of an apparent molecular mass of 180 kDa as determined by SDS-polyacrylamide gel electrophoresis. This activity, which we call DNA strand transfer protein beta (STP beta), has reaction properties similar to those of DNA strand transfer protein alpha (STP alpha) purified from crude extracts of yeast meiotic cells (Sugino, A., Nitiss, J., and Resnick, M. A. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 3683-3687). However, STP beta differs from STP alpha in its molecular weight and column chromatographic behavior as well as by immunological comparison. Furthermore, the STP beta polypeptide remains in cells in which the STP alpha gene has been disrupted. Thus, we conclude the STP beta activity is encoded by a gene different from that for STP alpha. Although STP beta was isolated from mitotic cells, the amount of STP beta increases severalfold during meiosis. STP beta also appears to differ in molecular weight from similar activities described by other groups and may be an intact form of their activities.