Identification of a protein coded by pR plasmid and involved in SOS repair in E. coli

Expression of genes carried by pR plasmid in damaged E. coli and mouse cells

In LTA mouse cells pR plasmid constitutively expresses itself resulting in protection against typical SOS inducers (UV, 4NQO) and in sensitization to different DNA-damaging agents (MNNG, cisDDP, BLM and geneticin (G418). The pR sensitizing effect is specific to mammalian cells, since the plasmid can only protect prokaryotic cells against the damaging agents tested. The pR protecting effect requires the expression of both the uvp1 and uvp2 (mucAB) regions in bacteria as well as in mouse cells. The coordinated function of these regions could result in protection against typical SOS inducers through an SOS/SOS-like pathway. The sensitization conferred by pR plasmid depends mostly on the expression of the mucAB genes, as shown by the survival of mouse cells transfected with different pR::Tn5 mutants. In particular, BLM and G418 survival data demonstrate that, inserted into the pR plasmid, the ble and neo genes of the Tn5 transposon express themselves. This was confirmed by the presence of Tn5 transcripts in untreated mouse cells. The comparison between the pR effects in bacterial and mouse cells shows that during evolution the repair pathways against UV damage are better conserved than those against other kinds of damage.

Plasmid pGW16, a derivative of pKM101, increases post-UV DNA synthesis, but sensitises some strains of Escherichia coli to UV

Plasmid pKM101, which carries muc genes that are analogous in function to chromosomal umu genes, protected Escherichia coli strains AB1157 uvrB+ umuC+, JC3890 uvrB umuC+, TK702 uvrB+ umuC and TK501 uvrB umuC against ultraviolet irradiation (UV). Plasmid pGW16, a derivative of pKM101 selected for its increased spontaneous mutator effect, also gave some protection to the UmuC-deficient strains, TK702 and TK501. However, it sensitised the wild-type strain AB1157 to low, but protected against high doses of UV, whilst sensitising strain JC3890 to all UV doses tested. Even though its UV-protecting effects varied, pGW16 was shown to increase both spontaneous and UV-induced mutation in all strains. Another derivative of pKM101, plasmid pGW12, was shown to have lost all spontaneous and UV-induced mutator effects and did not affect post-UV survival. Plasmids pKM101 and pGW16 increased post-UV DNA synthesis in strains AB1157 and TK702, whereas pGW12 had no effect. Similarly, the wild-type UV-protecting plasmids R46, R446b and R124 increased post-UV DNA synthesis in strain TK501, but the non-UV-protecting plasmids R1, RP4 and R6K had no effect. These results accord with the model for error-prone DNA repair that requires umu or muc gene products for chain elongation after base insertion opposite non-coding lesions. They also suggest that the UV-sensitizing effects of pGW16 on umu+ strains can be explained in terms of overactive DNA repair resulting in lethal, rather than repaired UV-induced lesions.

The uvp1 gene of plasmid pR cooperates with mucAB genes in the DNA repair process

We show that a DNA fragment that contains the uvp1 gene of the plasmid pR directs the synthesis in Escherichia coli minicells of a protein of apparent molecular weight 20 kDa. Inspection of the nucleotide sequence of the region reveals an open reading frame that has the capacity to encode a protein of 198 amino acids. The uvp1 gene product has been found, in two different systems, to enhance the recombinational activity of E. coli cells. We have also observed a striking similarity to resolvase and invertase proteins. The significance of this finding for the function of the uvp1 gene product requires further investigation. We conclude that the uvp1 gene encodes a 20 kDa protein which appears to be responsible for enhancement of both UV survival and recombinational activity in E. coli.

The rep region of pR plasmid regulates the expression of SOS system

By using an artificial hybrid between phage lambda and the pR plasmid, we have shown that the rep region of the pR plasmid encodes a function which regulates the expression of the muc genes (plasmid genes that are under the negative control of lexA and responsible for an increased rate of spontaneous mutagenesis and resistance to UV and chemicals). Expression of the muc genes were monitored by a fusion between the muc promoter and the lacZ structural gene. When E. coli cells containing such a fusion are infected by the hybrid lambda pR phasmid, beta-galactosidase activity is enhanced, indicating that pR encodes an antagonist of lexA. By deletion mapping we have located the gene encoding the antagonist of lexA (bat) in the rep region of the plasmid. The bat gene product can also antagonize the lambda cI repressor as shown by the observation that lambda pR phasmids are virulent on a homoimmune lysogen. We have exploited this latter property to carry out genetic and functional analysis of the bat region. This region is organized as a classical operon where the expression of the bat structural gene is negatively regulated by a repressor gene that encodes a proteic product.

The pR plasmid: a tool for discriminating between DNA lesions induced by different types of cytotoxic agents in cultured mammalian cells

The LA-D cells, obtained by cotransformation of LTA mouse cells (tk- aprt-) with pR plasmid and with tk gene as selective marker, are significantly more resistant to UV light and 4-nitroquinoline-N-1-oxide than LTA control cells. In this work, we report that the LA-D cells exhibit different degrees of response to various DNA-damaging agents: wild-type survival to mitomycin, increased sensitivity to bleomycin, cis-diamminedichloroplatinum and N-methyl-N'-nitro-N-nitrosoguanidine. The pR plasmid could, therefore, play an important role in the DNA-repair mechanisms that modulate the cytotoxic effect of the DNA-inhibitory agents. The possible interactions between pR plasmid products and the different repair enzymes involved are discussed.

Cellular response to DNA damage is enhanced by the pR plasmid in mouse cells and in Escherichia coli

The pR plasmid, which enhances the survival of Escherichia coli C600 exposed to UV light by induction of the SOS regulatory mechanism, showed the same effect when it transformed mouse LTA cells (tk-, aprt-). With Tn5 insertion mutagenesis which inactivates UV functions in the pR plasmid, we recognized two different regions of the plasmid, uvp1 and uvp2. These pR UVR- mutants exhibited the same effect in LTA transformed cells, demonstrating that resistance to UV light, carried by the pR plasmid, was really due to the expression of these two regions, which were also in the mouse cells. Statistical analysis showed that the expression of the uvp1 and uvp2 regions significantly increased (P less than 0.01) the survival upon exposure to UV light in mouse cells and bacteria. These results might suggest the presence of an inducible repair response to DNA damage in mouse LTA cells.

Cellular response to DNA damage is enhanced by the pR plasmid in mouse cells and in Escherichia coli

The pR plasmid, which enhances the survival of Escherichia coli C600 exposed to UV light by induction of the SOS regulatory mechanism, showed the same effect when it transformed mouse LTA cells (tk-, aprt-). With Tn5 insertion mutagenesis which inactivates UV functions in the pR plasmid, we recognized two different regions of the plasmid, uvp1 and uvp2. These pR UVR- mutants exhibited the same effect in LTA transformed cells, demonstrating that resistance to UV light, carried by the pR plasmid, was really due to the expression of these two regions, which were also in the mouse cells. Statistical analysis showed that the expression of the uvp1 and uvp2 regions significantly increased (P less than 0.01) the survival upon exposure to UV light in mouse cells and bacteria. These results might suggest the presence of an inducible repair response to DNA damage in mouse LTA cells.

The effects of cis-platinum (II)diamminodichloride, UV light and N-methyl-N'-nitro-N-nitrosoguanidine on Escherichia coli:plasmid mediated resistance to mutagens

Protease deficient recA431 mutants of Escherichia coli are defective in their capacity for induction of SOS responses and were intermediate in their sensitivities to ultraviolet light (UV) and cis-platinum (II) diamminodichloride (cis-PDD). Survival after treatment determined as colony forming ability was greater in rec+ strains and decreased in recA13 mutants which are defective in both recA proteolytic and recombination capabilities. In contrast, recA431 mutants were as sensitive to N-methyl-N'-nitro-N-nitrosoguanidine (NTG) as the recA13 cells. When cells carried either the pKM101 or N3 plasmid, survival after treatment with the three mutagens was increased. Presence of these plasmids in cells also resulted in hypermutagenicity as indicated by reversion of the argE3 mutation using a modified Ames test. Mutagenesis by NTG and cis-PDD was increased, as was survival of cells treated with UV light, cis-PDD and NTG in both recA+ and recA431 (protease deficient) strains. No plasmid mediated enhancement of mutagenesis or cell survival was observed in recA13 mutants. Thus, the ability of the plasmids to enhance cell survival and mutagenesis was dependent on recombination proficiency of the recA gene product and not its regulatory proteolytic activity. Unlike UV or NTG, presence of one of these plasmids was needed to detect reversion of the argE3 mutation by cis-PDD.

The pR UV+ plasmid, transfected into mammalian cells, enhances their UV survival

It has been recently reported that the pR plasmid enhances the UV survival in E.coli c600. In order to test whether this function may be expressed also in mammalian cells, LTA (tk- aprt-) mouse cells were cotransformed with pR plasmid DNA and ptk1 plasmid as selectable marker. Tk+ transformants were analyzed for their UV survival and for the presence of pR DNA sequences by blot-hybridization. The results show a correlation between the enhanced UV survival and presence of pR DNA sequences in cotransformed LTA mouse cells.

Tn5 mutagenesis of the transforming genes of human adenovirus type 5

We have cloned the entire human adenovirus type 5 (Ad5) genome into the pBR322 plasmid in two segments: the BamHI-A fragment (21 kb) and the BamHI-B fragment (15 kb). We have also generated a series of clones with smaller Ad5 DNA inserts, all containing the left-end of the viral genome. One such clone, pXCl, containing the left 16% of the Ad5 DNA molecule, has been shown to transform rodent cells by DNA transfection. We have used the transposable element Tn5 as an insertion mutator to isolate pXCl mutants containing Tn5 inserted at a large number of sites. By assaying transforming activity of selected pXC::Tn5 plasmids we have identified Ad5 sequences which are essential for DNA-mediated transformation. Our results with these mutants and with a plasmid pCD1, containing a deletion within the Ad5-transforming region, indicate that sequences present in both early region 1a and the N-terminal region of early region 1b are essential for DNA-mediated transformation.