Occurrence and diversity of plasmids in populations of streptomycetes in soil

Studies were made of naturally occurring plasmids hosted in Streptomyces st rains isolated from two different terrestrial ecosystems: an agricultural f ield and a protected forest area. Six out of the 147 screened isolates cont ained plasmids. The strains containing these plasmids were all isolated fro m the agricultural soil. Plasmids were not found among the strains isolated from the forest area. Cross hybridization of the six newly isolated plasmi ds revealed very high similarities between four of them. However, no simila rities were found between the six newly isolated plasmids and well studied streptomycete plasmids such as pIJ101 and SCP2*. The host strains of the fo ur similar plasmids belonged to three different species S. anulatus, S. roc hei and S. diastaticus. This implies a possible conjugative transfer of the se plasmids within the streptomycete population in the agricultural area. T he reason for the absence of streptomycete plasmids from the populations de rived from the forest area is discussed.