cslAsc gene replacement and genetic complementation. pHL151 was introduced into M145 from ET12567/pUZ8002 by E.coli-streptomyces intergeneric conjugation as described previously. Apr Hyg exconjugants were transferred to selective agar medium containing nalidixic acid and hygromycin,allowed to sporulate,and then transferred to MS agar without antibiotic selection spores from the MS agar were then plated on MS agar containing hygromycin to obtain single, which replicated on agar media containing only hygromycin and both apramycin and hygromycin. Five Aps Hyg colonies were obtained,all with the same phenotype on MS,MM,and R2YF agars. to confirm gene replacement in these isolates, southern blots were performed with a digoxi-genin-labeled 3.8 Kb BglII/PvuII fragment containing csLAsc as aprobe. complementation of the mutant wasachieved pHL155 by conjugation the vector pSET152 was also introduced into the mutant and M145 to provide negative controls.