The expression of Bacillus anthracis protective antigen (PA) in B. subtilis from the pag gene in pPA101-1 was explored in different genetic backgrounds in an attempt to identify opportunities to maximize expression. Introduction of AtxA, which positively regulates PA expression in B. anthracis did not improve expression levels in the protease-deficient strain WB600. Plasmid pPA101-1 was found to carry a deletion which created a new fusion point between vector and insert sequence, and which removed part of the AtxA binding site. The deletion may have occurred as a consequence of recombination between TCTAT sequences within both the vector and insert. Host mutations could influence expression; PA levels from pPA101-1 are threefold higher in a ccpA mutant than in an otherwise isogenic parent, and eightfold higher in an abrB mutant. These data demonstrate that the introduction of mutations affecting catabolite repression and growth phase regulation results in an increase in the yield of PA in this host-vector system. Combining these mutations with a multiply protease-negative background could potentially allow further improvements in PA yield.