Two linear deoxyribonucleic acid plasmids, designated pGKll and pGK12, were isolated from the yeast Kluyveromyces lactis IFO 1267. pGKll and pGK12 had molecular weights of 5.4 x 106 and 8.4 x 106, respectively. Both plasmids possessed the same density of 1.687 g/cm3, lighter than the densities of mitochondrial (1.692 g/cm3) and nuclear (1.699 g/cm3) deoxyribonucleic acids. A restriction map of pGKll was constructed from digestions by EcoRI, HindIII, PstI, and BamHI. pGK12 was cleaved by EcoRI into seven fragments and by BamHI into two fragments. K. lactis IFO 1267 killed Saccharomyces cerevisiae sensitive and killer strains and certain strains of Saccharomyces italicus, K. lactis, Kluyveromyces thermotolerans, and K. vanudenii. All K. lactis strains lacking the pGKl plasmids were nonkillers. A hybrid was constructed between K. lactis IFO 1267 and a nonkiller K. lactis strain lacking the plasmids and subjected to tetrad analysis after sporulation. The killer character was extrachromosomally transmitted in all tetrads in association with the pGKI plasmids. The double-stranded ribonucleic acid killer plasmid could not be detected in any K. lactis killer strains. It is thus highly probable that the killer character is mediated by the linear deoxyribonucleic acid plasmids. A single chromosomal gene was found which was responsible for the resistance to the K. lactis killer.

The terminal structure of a linear plasmid pSLA2, which was isolated from Streptomyces rochei, was analysed. The 5' ends of pSLA2 DNA were blocked by the association of a protein probably covalently bonded with the DNA. This block is removed by alkali treatment and blunt ends with 5' -phosphate and 3' -hydroxy termini were released. The two terminal fragments of pSLA2 were cloned and the nucleotide sequence was determined. An inverted terminal repetition of 614 bp was found along with the presence of further interrupted homologous sequences beyond this area up to 800 bp. These are the first inverted terminal repeat sequences found in microbial linear plasmids.

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