Some of the most important evolutionary events in Earth's history didn't just create new organisms -- they created new fundamental biochemical processes. And where do biochemical processes come from? They evolve from other biochemical processes.
Two of the most important pieces of biochemical innovation that occurred in the early biosphere -- the development of photosynthesis (which made light energy available to life) and of nitrogen fixation (which made atmospheric nitrogen available to life) -- may be related to each other because some of their key enzymes appear to have evolved from a common ancestor that may be part of a third, significantly different, biochemical process.
"Photosynthesis was important because it gave life an enormous energy source and ultimately put oxygen in the atmosphere," said Arizona State University biochemist Robert Blankenship.
"Nitrogen fixation -- making atmospheric nitrogen bioavailable -- was also a critical step in the early development of life. We need a good source of nitrogen for proteins and DNA, but the biggest source, the molecular nitrogen that we have in the atmosphere, has a triple bond in it that makes it so inert that it's a killer to get at."
Two new studies, to be presented at the February 2003 NASA Astrobiology Institute General Meeting by researchers at Arizona State University, provide evidence for the long-suspected relatedness of the two biochemical pathways, and find hints of other related pathways that may be key to understanding the evolutionary history of both.
A critical part of the emerging evolutionary picture seems to be "horizontal gene transfer" -- genetic change that occurs by the exchange of genetic material between bacteria. This process allows for sudden evolutionary leaps that are perhaps not possible through gradual genetic change and natural selection.
In a paper published in the November 22, 2002 issue of Science, Blankenship, ASU biochemist Jason Raymond and colleagues show through a comparative genomic analysis of five photosynthetic prokaryotic organisms that the genes that code for the intricate molecular complexes that perform photosynthesis seem to have originated through ancient genetic mixing that apparently combined a variety of independently evolved metabolic processes.
In one of the Astrobiology meeting papers, "Horizontal Gene Transfer in the Evolution of Nitrogen Fixation," Raymond, Blankenship and Rice University's Janet Siefert do an analysis of the genomes of a larger group of bacteria and archaea, comparing in particular similar genes that code for the protein nitrogenase, a critical enzyme in nitrogen fixation.
"In the very early earth, there was probably some available nitrogen in the form of ammonia or something else, so early life forms didn't have to fix nitrogen from the atmosphere.
"At some point though, things reached a food crisis -- you either find someway to get the atmosphere's molecular nitrogen into the cycle or you die. A minimum input of nitrogen can't sustain a big biosphere," noted Blankenship.
"Nitrogen fixation is one of the most interesting biological processes because it's so difficult to do chemically. Nitrogenase is a very complex enzyme system that actually breaks molecular nitrogen's triple bond," he said.
The researchers find that similar or "homologous" nitrogenase genes exist across a broad range of organisms, and appear to be related to other similar genes coding for proteins involved in photosynthesis, as well as to other genes in archaea and bacteria that do neither photosynthesis nor nitrogen fixation.
"We found a group of homologous genes that doesn't correspond to any genes that go with photosynthesis or any that we know in nitrogen fixation -- we found these in a wide range of organisms," said Raymond.
The analysis suggests that the related genes that code for neither nitrogenase nor enzymes in photosynthesis may be "relics," coding for metabolic pathways that are ancestral to both photosynthesis and nitrogen fixation.
Horizontal gene transfer appears to be responsible for the broad distribution of the original gene and for its subsequent divergence and specialization in the metabolic pathways of nitrogen fixation and photosynthesis.
In the second paper, "The Evolutionary Relationship between Nitrogen Fixation and Bacteriochlorophyll Synthesis," ASU's Christopher Staples, Blankenship, and Virginia Polytechnic Institute's Biswarup Mukhopadhyay examine the properties of enzymes created by these similar genes and finds that nitrogenase, the photosynthesis related enzymes, and other homologous enzymes all generally belong to a group of enzymes that break apart molecules and are known as reductase enzymes.
"We're purifying the proteins that the genes produce and will be looking at catalytic activity. We will test to see how activity differs and also to find what has been conserved and what has been changed in the active sites," said Staples. "Changes in the enzymes' active sites lead to differentiation in regard to what specific molecules they affect."
The less-specialized reductase enzymes appear to be ancestral to the others and were perhaps originally important in helping early prokaryotes neutralize toxic substances in their environment.
"There is a hypothesis that the ancient reductase, in the presence of a reducing atmosphere, may have been a hydrogen cyanide reductase," said Staples.
The team thinks that they have perhaps found a living model for this in Methanococcus jannaschii, a methane- producing archaea that performs neither nitrogen fixation nor photosynthesis but produces a reductase enzyme that the researchers suspect is used to break down hydrogen cyanide.
"We're testing to see if these organisms can grow in the presence of cyanide and if they can use cyanide as a nitrogen source," said Staples. "They don't appear to be able to use cyanide exclusively for nitrogen, but they can grow in concentrations of it that would be deadly to most organisms."
While the search to discover the evolutionary history of the key chemical processes of the biosphere involves some esoteric genomic and biochemical detective work, Blankenship, Raymond and Staples point out that understanding how the chemical processes of photosynthesis and nitrogen fixation evolved may have some large practical pay-offs.
"Understanding the origins of nitrogenase, for example, links to things like the synthesis of fertilizer," said Blankenship. "I come from the Midwest where there are these huge anhydrous ammonia plants that are tremendous users of energy -- a fantastic amount of energy goes into the making of ammonia. But that's exactly what this enzyme complex does: make ammonia out of nitrogen.
It's a bio solution to this incredibly important and very expensive process of fertilizer production.
"There's tremendous appeal in having a bioengineered version of nitrogen fixation. If we understand this complex pathway and its origin and evolution, then we can think more effectively about engineering it into other places. The benefit to society of being able to engineer in nitrogen fixation into most crop plants would be profound," he said.
Arizona State University Tempe
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Titan May Hold Clues To Origin Of Life
Tucson - Dec 12, 2002
Enshrouded in an atmosphere impenetrable to the visible light, Saturn's largest moon has never revealed its surface. No one has been able to see through the orange-brown atmospheric haze and admire the unknown world below. Still, researchers know that Titan is a planet-size organic reactor where "building blocks" of life are being generated as they might have been created 4 billion years ago on Earth.
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