Abstract

Two plasmids, pMH1 with 7 kilobase pairs (Kbp) and pM41 with 8 Kbp, were purified from Spiroplasma citri strains MH and M4, respectively, and characterized by restriction mapping. Upon in vitro DNA recombination with plasmid pBR328 as a vector, pMH1 was cloned in Escherichia coli. Radioactive probes specific of the plasmids were used to investigate the occurrence of pMH1 and pM41 DNA sequences among various spiroplasmas. pM41 or a closely related plasmid was found in three other S. citri strains and also seems to be present as an 8-Kbp plasmid in three spiroplasmas not belonging to the S. citri species. Up to now, pMH1 had been found as a free 7-Kbp plasmid only in the S. citri strain MH. However, DNA sequences corresponding to the entire pMH1 DNA have been found to be integrated in the high-molecular-weight-DNA molecules and perhaps the chromosomal DNA itself of two other S. citri strains. DNA sequences hybridizable with pMH1 DNA have also been found to be integrated into high-molecular-weight-DNA molecules from several spiroplasma strains not belonging to the S. citri species.