To survie, everything is possible

No question has been resolved but more questions come out.

1. Is it safe to say that pilQ and comEC are the only channels for DNA passing through the outer membrane and inner membrane now?

For the inner membrane comEC/rec-2 might not be the only channel through which exogenous DNA passes through. My previous experiment clearly shows that transformation did not reduced at all in comEC/ycaI homologue mutant. For the outer membran, pilQ homologue (hofQ and gspD) are not required and so the outer membrane is not exclusive.

2. Stress induce E. coli transformation or nutrient limitation induces this transformation?

Transformation was regulated by rpoS, so transformation should be a response to the general stress. My previous experiment also implicated that crp might play similar role in transformtion. Unfortunately, I did this experiment only one time and after that the mutant was contaminated and always after transformation only white colonies appear on plates and plasmid can not be isolated from these white colonies. So I am not very sure about this result. But I did this experiment with the positive control (Wild type), negative control (rpoS mutant). I used two independent markers to mark the transformant: ampicillin resistance and red fluorescence. Even only with one time result, still I think this should be true. It seems that transformation is regulated both by stress and nutrient limitation. I can just imagine what it might be but it is hard to prove anything since I do not know throug which channels DNA enter into cells.

3. the relation among competence, nutrient limitation and stress. Difficult to distinguish and why we distinguish them?

I repeated Finkel's experiment. Although they lack some essential controls, their conclusion that hofQ is required for DNA used as nutrient is true. I added a control in this experiment, that is let cells grow in minimum culture added glucose to see whether the mutant can eat glucose.

We proposed if hofQ mutant (cloramphnicol resistance) behave the same as wild type cell, when we put the same number of wild type cells and mutant cells in the culture supplemented with glucose, the number of cells on LB plate should be 2 times of that on LB supplemented with cloramphinicol. To our surprise, we get 0 clonies on the plates with chloramphicol and more than 1000 transformants on LB plates (culture was serial diluted). That is wild type cells gain growth advantage even in minimum culture with glucose (they have reported such phenomenon in LB long ago). And such phenomenon was also found in minimum culture added DNA.

Later, I found that hofQ mutant with cat gene cassette grow slower than that of wild type cells in minimum culture with glucose and after I delete the cat gene cassette away, the mutant return back to the normal growth. The mutant without cat cassette can not 'eat' DNA either. Kolter's group has reported that 10-day-old cell can suppress the growth of 1-day-old cell and this phenomenon was regulated by rpoS more than 15 years ago. It is possible that hofQ mutant with cat cassette grow slower (just like 1-day-old cell) and its wild type competitor (like 10-day-old cell) grow quicker and take grow advantage. These results implied that competence homologue gene might be involved in both nutrient acquisition and general stess resposne.

Recent opinions to the evolution of genetic transformation incline to the theory of genetic exchange to gain genetic diversity to cope with stresses in environment, although DNA nutrient theory has also been proposed by some people. In my opinion, the two theory should not conflict, considering that nutrient limitation should be the biggest stress for bacteria in environment.