THREE DISPARATELY REGULATED GENES FOR SIGMA(32)-LIKE TRANSCRIPTION FACTORS IN BRADYRHIZOBIUM JAPONICUM

摘要

Bradyrhizobium japonicum possesses a subclass of heat-shock genes whose members are transcribed from a sigma(32) consensus promoter. Having identified previously one gene (rpoH(1)) encoding a sigma(32)-like RNA polymerase transcription factor, we report here the characterization of two additional rpoH-like genes (rpoH(2) and rpoH(3)). B. japonicum thus represents the first example of an organism possessing an rpoH multigene family. All three rpoH genes encode functional proteins that are able to initiate transcription from the Escherichia coli groE promoter. Each rpoH gene is apparently regulated by a different mechanism. Although both rpoH(1) and rpoH(2) are transcribed from sigma(70)-type promoters, transcription of the rpoH(1) operon was found to be heat inducible by an unknown mechanism, whereas the level of rpoH(2) mRNA decreased after heat shock. At extreme temperatures (48 degrees C), rpoH(2) was transcribed from a second promoter that resembled the E. coli sigma(E)-type promoter. The rpoH(3) gene was found to be associated with two upstream genes, ragA and ragB, coding for a classical two-component regulatory system. Transcription initiated from a promoter that mapped in front of the putative response regulator gene ragA, suggesting that ragA, ragB and rpoH(3) are organized in an operon. The ragA promoter was similar to a sigma(32) consensus promoter. The three B. japonicum rpoH genes also varied in their significance to support growth of the organism. While the rpoH(2) gene could not be eliminated by mutation, knock-out mutants of rpoH(1) and/ or rpoH(3) were readily obtained and shown to be indistinguishable from the wild type under aerobic growth conditions or during root-nodule symbiosis. We conclude that rpoH, is essential for the synthesis of cellular proteins under physiological growth conditions, whereas rpoH(1), and probably also rpoH(3), are involved in their synthesis during the stress response. [References: 59]