Relaxed rrn expression and amino acid requirement of a Corynebacterium glutamicum rel mutant defective in (p)ppGpp metabolism

Abstract The stringent response in Corynebacterium glutamicum was investigated. Sets of rrn –cat fusions were constructed in their native chromosomal position to examine the effects of amino acid starvation in a rel+ strain and a Δrel mutant defective in (p)ppGpp metabolism. The expression of the six rrn operons in the rel+ control was stringently regulated and reduced to 79% upon induction of amino acid starvation. The Δrel mutant displayed a relaxed regulation and was unable to reduce the rrn expression under amino acid depletion conditions. In addition, the Δrel mutant grew more slowly in minimal medium than a rel+ control. This growth effect was restored by a plasmid-encoded copy of rel or, alternatively, by supplementation of the minimal medium with the amino acid mixture casamino acids. In particular, the Δrel strain of C. glutamicum displayed a requirement for the amino acids histidine and serine.