Phenotype of the rpoD mutant

This mutation causes H. influenzae cells to become competent prematurely, and to reach levels of competence in rich medium that are about 100 times higher than normal cells.  The mutation causes a single amino acid substitution in domain 3 of the 'housekeeping' transcription factor called 'sigma 70'.  rpoD is an essential genes, needed for transcription of most housekeeping genes.  Since the mutant strain (named RR753) shows only a very slight decrease in exponential growth rate we think the mutation causes only a very minor change in the protein's function

My earlier post this morning said I had never explained my hypothesis about how this mutation causes increased competence, but actually I did, briefly here.  Here's the key sentences from that post:

My hypothesis is that the mutation's effect on transcription of sxy mRNA increases competence by increasing sxy translation.   I've long hypothesized that slowing elongation or increasing pausing in the 100 nt segment of sxy mRNA that forms its regulatory secondary structure will promote sxy translation by increasing the ribosome's access to the sxy ribosome-binding site and start codon. 

Then I listed some low-tech phenotypic analyses that the senior of our two co-op technicians could do:

• Is RR753 sensitive to the inhibition of competence by added purines?
• What's the effect of an hfq deletion in this background?
• How does this strain respond to added cAMP?
• How does it respond to the standard competence-inducing MIV treatment?
• Does the mutation increase competence of a sxy mutant (sxy6) that has an extra-stable secondary structure?
• Does it further increase log-phase competence of the sxy hypercompetence mutants, which have weakened sxy mRNA secondary structures?

She's now done all but the last of these, and we're considering what she should do next.  So here we're going to summarize what she's found and what we think it means.

Her first experiments gave a better characterization of RR753's growth rate.  She's done both Bioscreen growth curves (high-precision analysis of exponential growth) and manual ones (lower precision but better for cultures at low and high densities).

First the Bioscreen results:  Here growth of RR753 (red line) is compared to wildtype cells (KW20) and two other hypercompetence mutants, RR563 (sxy-1) and RR749 (murE).  This clearly shows the slightly slower exponential growth of the rpoD mutant.

The Bioscreen analysis measures OD600, so it doesn't tell us about the actual numbers of viable cells. The tech has also done a number of manual growth curves, mostly as control parts of experiments examining other variables.  These agree with the Bioscreen results in showing usually a slightly slower exponential growth rate, and no obvious differences in later survival.  

Her next time courses replicated my earlier measures of transformation frequency.  It looks like the rpoD mutant differs from the other hypercompetence mutants (in sxy and murE) in having very low competence at very low cell density, perhaps as low as that of wildtype cells.  The other mutants at 100-1000-fold more competent than wildtype cells at very low cell density. The rpoD mutant may also become highly competent at lower cell densities than wildtype cells, but may not be any different than the other hypercompetent mutants - these data are hard to interpret.

What could be the significance of having very low competence at very low cell densities?  I'd been assuming that the moderate competence of the sxy-1 hypercompetent mutants at low cell density reflected a baseline level of sxy transcription and an increased efficiency of translation.  If the rpoD mutation acts as I've hypothesized, it should have the same effect.  Provided the cells are in real exponential growth, the cell density shouldn't matter.  Might the rpoD mutant have two different 'exponential' growth phases, one at very low cell density and another at moderately low cell density?

Is this an important issue?  It would be quite a bit of work to investigate carefully, so let's set it aside for now.

The next experiments analyzed the effects of adding cAMP (known to stimulate transcription of sxy) and AMP (known to reduce translation of sxy mRNA).  I'll leave these for the next post.