CHARACTERIZATION OF PRA, A GENE FOR REPLICATION CONTROL IN PSAM2, THEINTEGRATING ELEMENT OF STREPTOMYCES-AMBOFACIENS

pSAM2 is a genetic element found integrated in Streptomyces ambofacien s (B2) and additionally in a replicating form in two mutants B3 and B4 . The presence of the pSAM2 replicating form in these mutants was the result of mutations located on pSAM2 in the pra locus, named pra3 and pra4 respectively. The pra gene is not directly involved in replicatio n, but its inactivation led to the disappearance of the pSAM2 free for m; therefore, it was considered as a replication regulator. The pra3 a nd pra4 mutations were located in the pra promoter and were shown to b e point substitutions that increase the promoter strength, The replica tion regulator role of pra was demonstrated by the fact that its const itutive expression in cells harbouring pSAM2(B2), which is normally on ly integrated, led to the appearance of the pSAM2 replicating form, No rthern analysis showed that the pra gene transcript can be detected on ly for the replicating mutants B3 and B4 and that the three adjacent g enes korSA, pra and traSA were transcribed separately. As replication of pSAM2 is not needed for its maintenance but is an indispensable sta ge of its transfer, the pra gene, described formally as an activator o f pSAM2 replication, is patently involved in pSAM2 transfer.