It turns out that B. subtilis and B. mohavensis grown without oxygen can synthesize ribonucleotides but can't convert them into deoxyribonucleotides, because their ribonucleotide reductase enzyme needs oxygen to remove the oxygen from ribose, converting it to deoxyribose (this seems backwards, but trust me). You can't just feed them deoxyribose, because they can't add it to bases. You can feed them deoxyribonucleosides (deoxyribose plus base) because they have specific kinases that put phosphates onto deoxynucleosides. Or you can give them DNA. Salmon sperm DNA, E. coli DNA, synthetic DNA, it all works fine. One microgram DNA/ml increases growth about 20-fold. The DNA can't replace sugar in the medium so the cells aren't using it as an energy source (i.e. they're not burning the DNA for fuel), they're just using it as building blocks for their own chromosome replication.
Is 1 microgram/ml enough to grow lots of B. subtilis cells? Back of the envelope calculation: say 1 cell/5000kb x 10^12 kb/microgram x 1 microgram/ml = 2 x 10^8 cells/ml. Yes, that's just about right.
The authors see no problem with cells using DNA as food; they're microbial physiologists and engineers, and apparently haven't been indoctrinated with the dogma that DNA can only provide genetic information. They write:
"The requirement of some Bacillus strains for DNA or deoxyribonucleosides for anaerobic growth should not be unexpected given their ability to uptake DNA by way of the competence system (7). Also, B. subtilis can cannibalize sibling cells (12), which prevents sporulation during transient periods of nutrient starvation. The nutrients released from lysed cells would include DNA and/or deoxyribonucleosides that could be used as nutrients. Thus, it is likely that some bacilli would have evolved the ability to utilize DNA or deonyribonucleosides as a nutrient source."The situation is different in E. coli. You can't feed them deoxyribonucleosides because they don't have the requisite kinases, and you can't just feed them deoxyribonucleotides because they have to strip the phosphate off (converting them to deoxyribonucloesides) to get them across the membrane. But E. coli has another ribonucleotide reductase, one that doesn't need oxygen, so it can always make the deoxyribonucleotides it needs from the ribonucleotides it synthesizes. Most other bacteria, including H. influenzae and some isolates of B. subtilis, also have this anaerobic ribonucleotide reductase.
uptake used as a verb, oh dear!
ReplyDelete