Parallel conjugative mating experiments were also performed to evaluate if the genes proposed to be involved in the excision and circularization of 89K are also involved in the transfer process. As shown in Table 1 , an int deletion mutant was transfer-deficient as expected, while the transfer frequency of the Dxis 89K derivative was reduced by almost two orders of magnitude, proximal to the lower limit of detection in our mating system. These observations suggest that the 89K-encoded integrase and excisionase likely cooperatively generate the extrachromosomal circular form of 89K, which represents an intermediate required for the transfer process itself.