PRESYNAPSIS AND SYNAPSIS OF DNA PROMOTED BY THE STP-ALPHA AND SINGLE-STRANDED DNA-BINDING PROTEINS FROM SACCHAROMYCES-CEREVISIAE

Abstract

We previously purified an activity from meiotic cell extracts of Saccharomyces cerevisiae that promotes the transfer of a strand from a duplex linear DNA molecule to complementary circular single-stranded DNA, naming it Strand Transfer Protein .alpha. (STP.alpha.) (Sugino, A., Nitiss, J., and Resnick, M.A. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 3683-3687). This activity requires no nucleotide cofactor but is stimulated more than 10-fold by the addition of yeast single-stranded DNA-binding proteins (ySSBs). In this paper, we describe the aggregation and strand transfer of double-stranded and single-stranded DNA promoted by STP.alpha. and ySSB. There is a good correlation between the aggregation induced by various DNA-binding proteins (ySSBs, DBPs and histone proteins) and the stimulation of STP.alpha.-mediated DNA strand transfer. This implies that the stimulation of ySSBs and other binding proteins is probably due to the condensation of single-stranded and double-stranded DNA substrates into coaggregates. Within these coaggregates there is a higher probability of pairing between homologous double-stranded and single-stranded DNA, favoring the initiation of strand transfer. The aggregation reaction is rapid and precedes any reactions related to DNA strand transfer. We propose that condensation into coaggregates is a presynaptic step in DNA strand transfer promoted by STP.alpha. and that pairing between homologous double- and single-stranded DNA (synapsis) occurs in these coaggregates. Synapsis promoted by STP.alpha. and ySSBs also occurs between covalently closed double-stranded DNA and single-stranded linear DNA as well as linear double-stranded and linear single-stranded DNAs in the absence of any nucleotide cofactors.