AN UNUSUAL SUICIDAL INTERACTION IN ESCHERICHIA-COLI INVOLVING NUCLEOID PROTEIN H-NS

A conditional-lethal mutation (rpoB364) mapping to the gene that encod es the beta-subunit of RNA polymerase was obtained in Escherichia coli . This mutation caused cell filamentation at the restrictive growth te mperature and partial derepression of the osmotically regulated proU o peron at the permissive growth temperature. Even under the latter cond ition, transformants of the rpoB364 mutant strain carrying the plasmid vector pACYC184, but not those carrying other polA-dependent: multico py plasmids such as pACYC177 or pBR322, were killed in early stationar y phase; one class of suppressor mutants isolated as survivors within these transformant colonies were further derepressed for proU-lac expr ession, and the mutation in each of several independent clones of this class was mapped to hns, the gene that encodes the protein H-NS of th e E. coli nucleoid. The hns mutations did not suppress the conditional -lethal growth phenotype of the rpo,B364 mutant itself. On the other h and, intracellular overproduction of guanosine 3',5-bispyrophosphate (ppGpp) in the rpoB364 strain alleviated both the growth inhibition at the restrictive temperature and the pACYC184-mediated stationary-phas e lethality. Upon subcloning into pUC19 or into pACYC177, a 105-bp Xba I-HindIII fragment from pACYC184 was shown to be sufficient to confer the rpoB364hns(+)-dependent lethal phenotype. We suggest that the leve l in stationary-phase cultures of a gene product(s) that interacts wit h the pACYC184 DNA fragment is altered in the rpoB364 hns(+) derivativ e (compared to that in rpoB(+) or rpoB364 hns strains) and that this r esults in cell suicide.