Spx activates transcription initiation inT hroughout phylogeny, positive transcriptional control plays a important role in cellular decision-making (45). The mechanisms that activate transcription initiation in response to environmental and metabolic signal inputs are varied and involve complex sensory functions linked to specific macromolecular interactions conducted at targeted chromosomal loci. For many regulatory systems, these interactions involve contacts between transcriptional regulators and RNA polymerase (RNAP). In prokaryotes, RNA polymerase, composed of , =, , 2 ␣, and subunits, contains multiple target surfaces that can engage positive regulatory proteins, which function to direct RNAP to the specific regulatory regions of genes that are under their control (17). The ␣ subunit, bearing two domains that occupy the protein's N and C termini (NTD and CTD, respectively), is a common target for regulatory protein interaction (18). A classic example is the class I and II positive control exerted by the cyclic AMP (cAMP) receptor protein-cAMP complex (CRP-cAMP), which interacts with a specific cis-acting element upstream of promoter DNA as a dimeric complex that recruits RNAP by direct interaction with RNAP ␣CTD (4).While direct interaction between DNA-bound regulators and RNAP is a common mechanism of positive transcriptional control, mechanisms of RNAP appropriation and "prerecruitment" have been reported that involve initial RNAP-regulator interaction prior to promoter engagement (2). Proteins encoded in bacteriophage genomes, such as NS4SSB of phage N4 (26) and AsiA of phage T4 (15), target RNAP without contacting DNA and yet participate in gene-specific transcriptional activation. The SoxS protein that mediates the bacterial response to superoxide, and the MarA protein that functions in control of multidrug resistance mechanisms, first contacts the RNAP ␣ subunit prior to sequencespecific DNA interaction (11,25). Spx, a protein of low-GC-content Gram-positive bacteria that serves as a transcriptional activator of genes that function in thiol-specific oxidative stress (11,25,51), also exerts control of RNAP through a prerecruitment mechanism (31).Spx governs a large regulon of genes that includes those encoding thioredoxin (trxA), thioredoxin reductase (trxB) (34), and methionine sulfoxide reductase (49) and genes whose products function in bacillithiol biosynthesis (9), cysteine biosynthesis (35), and iron uptake and metabolism (52). Spx has also been linked to control of biofilm-and virulence-related functions (19,39). Spx is under multiple levels of control that operate at the transcriptional and posttranscriptional level. Multiple forms of RNAP target the operon in which the spx gene resides (8,41). The spx gene itself is under negative transcriptional control involving two repressors, YodB and PerR, that control the transcriptional response to toxic electrophiles and peroxide, respectively (23). Spx protein is under tight proteolytic control involving the ATP-dependent protease ClpXP (35, 36) an...