Although heterospecific transformation is extremely inefficient and very little heterologous donor DNA integrates into the recipient chromosome in a stable way, we have previously shown that B. pumilus DNA entering competent B. subtilis efficiently associates with the recipient chromosome in an unstable way. This association can be stabilized by photocrosslinking in the presence of 4,5',8-trimethylpsoralen; it depends on the recombination proficiency of the recipient strain and on strand-separation of the recipient chromosome (te Riele and Venema 1982b). The present study provides further evidence that the heterologous donor DNA and the recipient DNA are associated by regions of base-pairing. Based on the high sensitivity of the donor moiety in the complex to nuclease S1 (90%) and the high sensitivity of the complex to moderate denaturing conditions (Tm = 48 degrees C), we presume that donor and recipient DNA are associated either by several short sequences of 15-25 fairly well matched base pairs or by a region of base-pairing of about 200 bases, which contains 25% of mismatches. During incubation, the unstable complex disappears, probably due to nucleolytic degradation. The unstable heterologous donor-recipient complex (DRC) was found to be membrane-bound. However, in contrast to homologous DRC, the unstable heterologous DRC remains membrane bound during incubation. Apparently, the predominantly single-stranded character of the heterologous DRC prevents release of the complex from the membrane.