To introduce DNA into Streptomyces noursei xinao-4, which produces xinaomycins, we explored an intergeneric conjugal transfer system. High efficiency of conjugation (8 x 10(-3) exconjugants per recipient) was obtained when spores of S. noursei xinao-4 were heat-shocked at 50 degrees C for 10 min, mixed with Escherichia coli ET12567 (pUZ8002/pSET152) in the ratio of 1: 100, plated on 2CMY medium containing 40 mmol/L MgCl2, and incubated at 30 degrees C for 22 h. With this protocol, the plasmids pKC1139 and pSET152 were successfully transferred from E. coli ET12567 (pUZ8002) with different frequencies. Among all parameters, the ratio of donor to recipient cell number had the strongest effect on the transformation efficiency. In order to validate the above intergeneric conjugal transfer system, a glycosyltransferase gene was cloned and efficiently knocked out in S. noursei xinao-4 using pSG5-based plasmid pKC1139.