TRANSCRIPTIONAL ANALYSIS OF THE CAULOBACTER 4.5-S RNA FFS GENE AND THE PHYSIOLOGICAL-BASIS OF AN FFS MUTANT WITH A TS PHENOTYPE

A temperature-sensitive (ts) mutation in the ffs gene, encoding 4.5 S RNA, gives rise to cell division and DNA replication defects in Caulob acter crescentus. The ffs gene is transcribed throughout the cell-cycl e and is transcribed at similar rates in mutant (ffs 36) and wild-type strains, but in the mutant the 4.5 S RNA is unstable leading to lower 4.5 S RNA levels. The ffs36 phenotype results from a single base chan ge in one of the non-conserved stems of the mature RNA, and is complet ely rescued by a compensating mutation in the opposite strand, providi ng confirmation of the predicted secondary structure of the 4.5 S RNA. The Caulobacter ffs gene was shown to be functionally comparable to t he Escherichia coli ffs gene by complementation. Comparison of the ffs 36 strain to a ts secA strain of Caulobacter, also having cell-cycle a nd DNA replication phenotypes, showed that both exhibit a permanent in duction of a heat shock response at the restrictive temperature. To ex plain the phenotype of both the secA and ffs36 strains, we propose tha t a cell-cycle checkpoint prevents further progression through the cel l-cycle in response to increased intracellular levels of heat shock an d misfolded proteins. (C) 1997 Academic Press Limited.