A couple of months ago I posted about the 'fraction competent' (FC) assay we use to tell whether only some of the cells in a culture are able to take up DNA. I mentioned that, when we assay cultures that are only partially competent (i.e. have lower transformation frequencies than maximally-induced cultures), we find this to be because some of the cells are taking up multiple DNA fragments and the rest aren't taking up any at all.
This was surprising, as I'd expected that the low transformation frequencies of such 'partially-induced' cultures would be because the cells were all only a little bit competent. We see this max-or-nothing pattern in not only in wild-type cells under poorly-inducing conditions, but also in low-competence mutants under fully inducing conditions and in hypercompetent mutants under what are otherwise non-inducing conditions. I've had this puzzling result hanging around in the back of my brain for about 15 years.
I've always thought that it reflected something about the action of adenylate cyclase, CRP or Sxy, the proteins whose actions control expression of all the genes in the competence (CRP-S) regulon. But yesterday we were talking about how the PurR repressor represses only one of the competence genes (rec-2), and I realized that, because the fraction-competent assays measure only transformation, the max-or-nothing could reflect the activity of a single gene instead or the whole regulon.
So maybe all the cells in our partially-induced cultures have turned on all the competence genes except rec-2, but only in some of them have levels of purines fallen low enough to inactivate the PurR repressor and turn on rec-2. This might be tested by repeating the FC assays on cells whose purR gene is knocked out. But first I need to check the notes from the grad student who created the purR knockout, to see if he already tested this.
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