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Why have nitrogen-fixing bacterial endosymbionts of plants not evolved into organelles (“azoplasts”)? (Nitrogenase’ sensitivity to oxygen is not a satisfying answer. Rhizobia have solved that problem!)
Posted on December 01, 2006 at 04:11 PM in Talmudic Questions, Teachers Corner | Permalink
In order to become an organelle, wouldn't the endosymbiont first have to "give up" most of its genome to the plant cell? Because, organelles might have one little plasmid of DNA, but they are not free-living inside their cells. Also, how would it be possible for the circular bacterial chromosome to be taken up into the linear chromosomes of a plant cell? I know that theoretically it can occur, especially in a lab. But in nature, after plants have been evolved to deal with linear chromosomes and have been doing so for ages, isn't it a little late? I mean, when mitochondria and chloroplasts became organelles, it was obviously very early in evolutionary history, possibly during a transition stage of cells when they actually could have had circular chromosomes. This would have made the process infinitely easier to occur naturally. I just can't see that happening in nature during this time period. May be, if the world exploded in nuclear war, the radiation would revert genomes back into circular DNA, and a whole slew of new organelles could arise...
Would that every student in their first microbiology class were as enthusiastic and questioning as you! Indeed, the endosymbiont has to give up much to become an organelle. Some of its genes are transferred to the nuclear chromosomes of the host, others are completely lost. Circular chromosomes insert into linear ones readily. Plasmids and phage, for example, do this all the time. You can see how by drawing this out on paper. With the circular chromosome next to the linear one, make a break in both chromosomes and rejoin each end of the circle to one of the ends of the linear chromosome. Voila! There are some other related posts on our blog that you might find interesting: Play It Again, Cyan, Some Like It Linear, and (for a mitochondrial digression), From Free-Living Bacterium to Cellular Citizen.
Autumn Cochrane |
June 27, 2008 at 10:44 AM
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