Suppression of the thermosensitive DNA ligase mutations in Escherichia coli K12 through modulation of gene expression induced by phage Mu.

We have previously shown that Mu can sustain the growth at non-permissive temperature of an Escherichia coli strain harbouring a thermosensitive mutation in the DNA ligase structural gene. This "complementation" reaches a maximal level with the Mu lig3 mutant which restores the viability of a ligts7 strain to the level of the wild type (Ghelardini et al. 1980; Paolozzi et al. 1980). In this study we analysed the characteristics of this phenotypic suppression in order to clarify its molecular mechanism. We found that an E. coli ligts7 strain lysogenic for the Mu lig3 mutant shows: (i) an increment in the host DNA ligase activity; (ii) an increase in the specific mRNA of the host lig gene; (iii) an increase (towards the relaxed state) in the average linking number of a resident plasmid; and (iv) a reduction in DNA gyrase activity. These results are compatible with the hypothesis that the Mu lig gene product by interfering with the host enzymatic apparatus controlling DNA topology leads to a reduction in chromosomal supercoiling. The relaxation of the chromosome could affect the transcription of the DNA ligase gene, amongst others. Thus, through this mechanism, the Mu lig gene product is able to modulate gene expression and hence suppress the effects of the E. coli ligts7 mutation. On the basis of the identification of this mechanism of action, we propose to change the name of the Mu lig gene (thought originally to be the structural gene for a bacteriophage ligase) to gem (gene expression modulation).