RecA protein catalyzes the homologous pairing of a single-stranded circular DNA and a linear duplex DNA molecule. When the duplex is packaged into chromatin, formation of homologously paired complexes is blocked. We have established a system for studying the RecA-promoted reaction by using a duplex fragment containing a single-phased nucleosome. Under these conditions there is no reaction leading to formation of joint molecule complexes. However, transcription on the chromatin template activates the formation of complexes. Reaction is dependent on RNA synthesis and DNA sequence homology and proceeds regardless of the direction of transcription.In general, chromatin is viewed as a repressive structure, down-regulating gene expression primarily by reducing the accessibility of trans-acting factors to their cognate binding sites (3,8,10,27). Repression extends to other processes such as genetic recombination, for which it has been observed in vitro that strand exchange promoted by Escherichia coli RecA protein is inhibited by chromatin (20). This raises the question of how recombination occurs in eukaryotes, in which genomes are packaged into chromatin. One notion that seems reasonable is that the homologous pairing and strand exchange machinery could gain access to DNA if chromatin structure were disrupted. DNA replication might be considered one means by which chromatin structure is disrupted. Here the helix is unwound and the relationship between the histones and DNA is altered (see reference 2 and references therein). Transcription is another process which, intuitively, would seem likely to destabilize or alter chromatin structure. Unwinding of the duplex resulting from binding by transcription factors or from active transcription might be supposed to disrupt chromatin structure sufficiently that recombination could be initiated.The activity of RNA polymerases on chromatin templates and the passage of RNA polymerase through nucleosomal particles has been the subject of intense investigation over the last several years. RNA polymerases from bacteriophages T7 and SP6 have been observed to transcribe through a nucleosome core assembled on a plasmid DNA sequence (13) or on a 5S RNA gene fragment (14). In similar in vitro studies with the eukaryotic enzyme RNA polymerase II or RNA polymerase III, transcriptional elongation was found to be inhibited by nucleosomes (16). The fate of the nucleosomes after polymerase transit through a nucleosome is a matter of controversy (4). In one well-studied case, nucleosomes were found to be completely displaced from a DNA template (13), whereas in another no net displacement was observed (14