The chromosomal DNA of Streptomyces hygroscopicus 10-22, a derivative of strain 5102-6, was digested with several restriction endonucleases and analyzed by pulsed-field gel electrophoresis (PFGE). Digestions with AseI gave 11 fragments with a total length of ca. 7.36 Mb. The AseI sites were mapped by analysis of overlapping chromosomal deletions in different mutants and confirmed by Southern hybridizations using partially digested genome fragments and linking cosmidsas probes. PFGE analysis of DNA with and without proteinase K treatment, together with the hybridization results, suggested a linear organization with terminal proteins and large terminal inverted repeats. Some deletion mutantshad circular chromosomes.

Several independent gene clusters containing varying lengths of type I polyketide synthase genes were isolated from 'Streptomyces nanchangensis' NS3226, a producer of nanchangmycin and meilingmycin.The former is a polyether compound similar to dianemycin and the latter is a macrolide compound similar to milbemycin, which shares the same macrolide ring as avermectin but has different side groups. Clusters A-H spanned about 133, 132, 104, 174, 122, 54, 37 and 59 kb, respectively. Two systems were developed for functional analysis of the gene clusters by gene disruption or replacement. (1) Streptomyces phage ΦC31 and its derived vectors can infectand lysogenize this strain. (2) pSET152, an Escherichia coli plasmid with ΦC31 attP site, and pHZ1358, a Streptomyces-Escherichia coli shuttle cosmid vector, both carrying oriT from RP4, can be mobilized from E. coli into NS3226 by conjugation. pHZ1358 was shown to be generally useful forgenerating mutant strains by gene disruption and replacement in NS3226 as wellas in several other Streptomyces strains. A region in cluster A (~133 kb) seemed to be involved in nanchangmycin production because replacement of several DNA fragments in this region by an apramycin resistance gene [aac3(IV)] gave rise to nanchangmycin non-producing mutants.