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

摘要

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.