Influence of conditioned medium on the constitutive SOS response in recombinational mutants of Escherichia coli

SOS odgovor je inducibilni sustav popravka DNA. Signal za indukciju SOS odgovora je RecA filament koji stimulira autoproteolitičko cijepanje represora LexA, nakon čega dolazi do ekspresije SOS gena. RecA filament nastaje djelovanjem helikazne i egzonukleazne aktivnosti i nanošenjem proteina RecA na ssDNA. U RecBCD putu ove aktivnosti osigurava enzim RecBCD, a u RecF putu helikaza RecQ, egzonukleaza RecJ i kompleks RecFOR koji nanosi protein RecA. Neki mutanti bakterije Escherichia coli (npr. recA730 i recB1080) konstitutivno eksprimiraju SOS gene kao posljedicu nagomilavanja oštećenja DNA zbog nemogućnosti njihovog popravka. Cilj ovog diplomskog rada bio je istražiti utjecaj iskorištenosti hranjive podloge na konstitutivnu SOS ekspresiju u različitim mutantima bakterije Escherichia coli mjerenjem aktivnosti β-galaktozidaze. Pokazano je da iskorištenost hranjive podloge: (1) povećava razinu konstitutivnog SOS odgovora u rekombinacijski proficijentnim sojevima, (2) povećava generacijsko vrijeme rekombinacijski proficijentnih i deficijentnih sojeva, (3) ne utječe na razinu konstitutivnog SOS odgovora u rekombinacijski deficijentnom soju recB21 te (4) ne utječe na vijabilnost ni rekombinacijski proficijentnih ni deficijentnih sojeva.SOS response is the inducible system of DNA repair. SOS response induction signal is the RecA filament which stimulates autoproteolitic cleavage of the LexA repressor and therefore the expression of SOS genes. The RecA filament is produced by the helicase, nuclease and RecA loading activities. In the RecBCD pathway these activities are provided by the RecBCD enzyme and in the RecF pathway by the RecQ helicase, RecJ exonuclease and RecFOR complex which has the RecA loading activity. Some E. coli mutants (e.g. recA730 and recB1080) exhibit constitutive SOS expression as a consequence of accumulation of DNA damage due to inefficient repair. The objective of this graduate thesis was to test the influence of conditioned medium od the constitutive SOS expression in different recombinational mutants of E. coli by measuring the β-galactosidaze activity. It was shown that conditioned medium: (1) increases the level of the constitutive SOS expression in recombination-proficient E. coli strains, (2) increases doubling-time of recombination-proficient and of recombination-deficient E. coli strains, (3) has no effect on the constitutive SOS expression in recombination-deficient E. coli recB21 strain and (4) has no effect on viability of recombination-proficient and in recombination-deficient E. coli strains