Mysterious old DprA data

I'm preparing a talk on "Do Bacteria Have Sex" for a symposium celebrating the 300th birthday of Linnaeus. While looking through old PowerPoint slides for ones appropriate for this topic and this audience and this amount of time, I found this slide, from a 2004 talk.

I don't know whether I'd forgotten about this data because subsequent experiments proved it wrong, or just because I'm forgetful. I've written a couple of posts about the DprA protein (here and, more recently, here) - the main issue is that it protects DNA from degradation, but we don't know how or why.

I do remember doing these and related experiments; I just don't remember getting this very interesting result. The logic is as follows: The recBC genes specify a nuclease that degrades DNA. The degradation can facilitate recombination, which is how the nuclease was discovered and why the genes have 'rec' names, but its primary function is to help resolve stalled/tangled replication forks. If DprA's job is to protect DNA from the RecBC nuclease; then cells that lack the recBC gene shouldn't need DprA. The experiment shows that this is the case; the transformation frequency of the recBC dprA double mutant is much higher than that of the dprA single mutant, even though on its own the recBC mutation does decrease transformation slightly. In genetic terms, the recBC mutation is epistatic to the dprA mutation (it covers up the dprA phenotype).

Now I really need to go back to my 2003/04 lab notebook to find out exactly what I did and what I thought it meant.