ABSTRACTArsR is a well-studied transcriptional repressor that regulates microbe-arsenic interactions. Most microorganisms have anarsRgene, but in cases where multiple copies exist, the respective roles or potential functional overlap have not been explored. We examined the repressors encoded byarsR1andarsR2(ars1operon) and byarsR3andarsR4(ars2operon) inAgrobacterium tumefaciens5A. ArsR1 and ArsR4 are very similar in their primary sequences and diverge phylogenetically from ArsR2 and ArsR3, which are also quite similar to one another. Reporter constructs (lacZ) forarsR1,arsR2, andarsR4were all inducible by As(III), but expression ofarsR3(monitored by reverse transcriptase PCR) was not influenced by As(III) and appeared to be linked transcriptionally to an upstreamlysR-type gene. Experiments using a combination of deletion mutations and additional reporter assays illustrated that the encoded repressors (i) are not all autoregulatory as is typically known for ArsR proteins, (ii) exhibit variable control of each other's encoding genes, and (iii) exert variable control of other genes previously shown to be under the control of ArsR1. Furthermore, ArsR2, ArsR3, and ArsR4 appear to have an activator-like function for some genes otherwise repressed by ArsR1, which deviates from the well-studied repressor role of ArsR proteins. The differential regulatory activities suggest a complex regulatory network not previously observed in ArsR studies. The results indicate that fine-scale ArsR sequence deviations of the reiterated regulatory proteins apparently translate to different regulatory roles.IMPORTANCEGiven the significance of the ArsR repressor in regulating various aspects of microbe-arsenic interactions, it is important to assess potential regulatory overlap and/or interference when a microorganism carries multiple copies ofarsR. This study explores this issue and shows that the fourarsRgenes inA. tumefaciens5A, associated with two separatearsoperons, encode proteins exhibiting various degrees of functional overlap with respect to autoregulation and cross-regulation, as well as control of other functional genes. In some cases, differences in regulatory activity are associated with only limited differences in protein primary structure. The experiments summarized herein also present evidence that ArsR proteins appear to have activator functions, representing novel regulatory activities for ArsR, previously known only to be a repressor.
THE ROLE OF MOLYBDENUM IN NITRATE ASSIMILATION BY AGROBACTERIUM TUMEFACIENS