pKM101, a plasmid R factor of the N compatibility group increases methylmethane sulfonate mutagenesis and diminishes UV-killing in recA+ LEX+ and recA+ lex- strains, , but not in recA-lex+ strains. The induction of a "reclex" dependent colicin is not present in lex- strains carrying the pKM101 factor. These facts indicate that pKM101 acts through an error-prone DNA repair system which is recA+ dependent, but not lex+ dependent.

A number of transition metal complexes, including the cis and trans isomers of dichlorodiammineplatinum (II), six complexes of rhodium (I), two of iridium (I), and one of ruthenium (II) have been tested for their ability to induce lambda prophage, to produce filamentous growth of Escherichia coli , and to be selectively toxic for strains with defects in the deoxyribonucleic acid repair system. Dichlorotetrakis(dimethylsulfoxide)ruthenium II [RuCl 2 (DMSO) 4 ] was strictly similar to cis -dichlorodiammineplatinum II [ cis PtCl 2 (NH 3 ) 2 ] in the test for lambda induction, filamentous growth production, and selective toxicity for a recA − strain. [Rh COD 1,10-phenanthroline] + Cl − , though more toxic for recA − than for rec + E. coli , was scarcely effective in the test for filamentous growth and did not induce prophage. None of the other tested compounds showed any similarity with cis -PtCl 2 (NH 3 ) 2 . Due to the interesting results obtained with cis -PtCl 2 (NH 3 ) 2 as an antitumor agent, it seems reasonable to propose RuCl 2 (DMSO) 4 as a potential antitumor substance.

The transmissible nature of the genetic factor in Escherichia coli, which controls a-haemolysin production, was recognized by Smith & Halls (1967). The element responsible was termed Hly. The f i f character of Hly factor was described at the same time. Goebel & Schrempf (1971) demonstrated the presence in the cytoplasm of a haemolytic E. coli strain of a covalently-closed circular DNA molecule with a contour length of 29.5 ? 0.5 pm, which was transferred to non-haemolytic strains together with the haemolytic character. Therefore this DNA species represents the Hly factor. However, Hly factor was not taken into account by Hedges & Datta (1972), who classified the F-like plasmids into four compatibility groups. A further group of F-like plasmids was subsequently added by Naomi Datta in 1974 (personal communication), Folac being the prototype of it (Falkow & Baron, 1962). In the present paper we show that Hly factor determines the production of F-type pili and that it is compatible with F-like plasmids of all the five groups previously described. Escherichiu coli strain ~2 1 2 carrying Hly factor was received from H. Williams Smith. Escherichia coli strains ~5 3 and c600, carrying many known plasmids, were provided by Naomi Datta. R7, R89 and R120, three F-like R factors recently identified in Triest, were also used. Crosses were carried out by mixing equal numbers (about 2 x 107/ml) of donor and recipient cells in nutrient broth, which was subsequently incubated overnight at 37 "C. Colonies were then counted on nutrient or blood agar containing, if required, appropriated amounts of different antibiotics to define the number of donor, recipient and recombinant cells. Colonies producing colicin were identified by standard techniques. To test for the compatibility of plasmids, recombinant clones were purified on nonselective medium and tested by replica plating for the presence of each plasmid. The fif character of Hly factor was confirmed in our study. Moreover, we could show that E. coli strains carrying the Hly factor allowed a small but definite increase in the titre of MS2 phage (Davis, Strauss & Sinsheimer, 1961). This demonstrates that Hly controls the synthesis of F-pili. Being not perfectly clear from the data reported by Smith & Linggood (1970) and Smith & Heller (1973) whether Hly is self-transmissible or mobilized by the F factor, we tested different strains of E. coli K I 2 F-, which had acquired the Hly factor from E. coli strain ~2 1 2 , for sensitivity to the female-specific phages T7 and $2 (Makela, Makela & Soikkeli, 1964; Cuzin, 1965). The Hly-positive bacteria remained sensitive to these phages. Hly factor was transmissible from these bacteria by conjugation. It appears, therefore, that at least the Hly factor used in the present study, for which we propose the name of Hly-Pzrz, is selftransmissible.

Daunomycin and adriamycin, are more mutagenic and antibacterial for a strain of Salmonella typhimurium defective for the uvrB gene than for its uvr+ counterpart. Other intercalating agents, as some acridine dyes, affect equally the two bacterial strains.

pTM89, an fi+ R factor that controls the production of repressed F-type pili, is incompatible with plasmids belonging to the FII and P groups. The results of P1 transduction show that all of the resistance markers of pTM89 are part of a single replicon, which also includes RTF. When the compatibility of different derivative plasmids was investigated, it was found that they fall into two classes. Those of the first class have lost the compatibility of pTM89 for the P group but are still incompatible with FII group, whereas those of the second class are compatible with plasmids of both groups. Plasmids of the latter class that are also compatible with each other and, therefore, apparently lack any determinant for compatibility are genetically stable and self-transmissible. It appears, therefore, that compatibility between plasmids cannot be explained by the hypothesis of competition for a maintenance site.