Binding of Glutathione and ppGpp to Stringent Starvation Protein A (SspA)

Abstract

Stringent starvation protein A (SspA) is a glutathione S-transferase homolog. In this study, his6-tagged SspA from Escherischia coli has been cloned and over-expressed. SspA binds glutathione and 1-chloro-2,4-dinitrobenzene, the substrates for glutathione S-transferases, with the dissociation constants as 225.0 ± 34.4 μM and 75.3 ± 4.3 μM, respectively. This observation is contradictory to the previous report that SspA, lacking glutathione S-transferase activity, does not bind glutathione. It has been reported that SspA is an RNA polymerase-associated transcription factor and that a functional relA gene is required for SspA to affect gene expression. A function of relA is to synthesize ppGpp, a global regulator in replication, transcription, and translation. This study shows for the first time that SspA binds ppGpp with the dissociation of constants of 109.1 ± 7.2 μM. This study may provide an insight why relA is required for regulating gene expression by SspA.