First the effect of adding cAMP: We tested this by adding 1 mM cAMP to cells growing exponentially at an OD600 of 0.1, and measuring transformation 60 min. later. At this growth stage, normal cells do not transform detectably, but addition of cAMP turns on sxy transcription. Some of the resulting sxy mRNA is translated and the Sxy protein acts with CRP to stimulate transcription of genes encoding the DNA uptake machinery. In the tech's experiment, cAMP addition raised the transformation frequency about 500-fold, from 1-3 x 10^-8 (just at the detection limit) to 6.5-8 x 10^-6. The rpoD mutant is somewhat transformable even with out cAMP (01-3 x 10^-6), and cAMP addition raised this about 100-fold, to ~2 x 10^-4.
So we conclude that the rpoD mutant does not bypass the need for cAMP in competence induction. This rules out the boring hypothesis that changing Sigma 70 activity perturbs cellular metabolism, causing an elevation of baseline cAMP levels that in turn causes the mutant's increased competence. Instead it's consistent with our interesting hypothesis that the rpoD mutation likely changes one or more events after the sxy transcription is stimulated by cAMP and CRP.
Next, the effect of adding AMP: Maximal competence is normally induced by transferring exponentially growing cells from rich medium to a 'starvation' medium called 'MIV' and incubating them for 100 min. Previous work has shown that adding purine nucleotides or nucleosides (usually 1 mM AMP) to the MIV prevents normal competence development by reducing the translation of sxy mRNA (Sinha et al. 2013). The next experiments tested whether AMP has the same effect in the rpoD mutant.
Both wildtype and rpoD mutant cells have high transformation frequencies after incubation in MIV. In these experiments the rpoD cells had slightly higher transformation frequencies (about 4 x 10^-3) than the wildtype cells (about 1 x 10^-3). Adding AMP to the MIV used to induce competence reduced the transformation of wildtype cells more severely than seen in previous work, about 5000-fold (from 1.7 x 10^-3 to 3.4 x 10^-7) and at least 10,000-fold (from 3 x 10^-4 to 3 x 10^-8). The AMP also reduced the viability of the cells by several fold. (Both replicates gave no transformants at all with added AMP, so these estimates are upper limits.)
These numbers are all lower than previous results, but the conclusion is clear that adding AMP to the MIV medium strongly inhibits the development of competence in the rpoD mutant. We don't know how the added AMP caused the reduction in competence, but, based on other evidence from analysis of purine-biosynthesis mutants, I've hypothesized that the key factor is a decrease in the concentration of another metabolite (PPRPP), which maybe interacts with sxy mRNA. Production of Sxy by the rpoD mutant is still sensitive to this effect, so... (I don't know what).