生物時計の研究
Welcome to SCIEN-SPOT. SCIEN-SPOT is a podcast that shines the spotlight on the latest in science
and technology from Japan. Your host is REN from SCIEN-TALK. Today's topic is the oldest biological
clock. We humans have biological clocks, but what about the very first one on Earth? When and how
did it emerge? And how was it connected to the Earth's environment at that time?
We are going to discuss truly fascinating research results that shed light on these questions.
This research was conducted by a group from Fukui Prefectural University, the National
Institute of Natural Sciences, Institute for Molecular Science, Nagoya University,
Osaka University, and the Graduate University for Advanced Studies.
And it has been published in the international academic journal Nature Communications.
First, let's talk a little about biological clocks, or circadian clocks. A biological
clock is a mechanism that living organisms use to measure time. Humans have them, of course,
but so do animals, plants, and even bacteria, including the main subject of our story today.
They typically exhibit a rhythm with a period of about 24 hours. This period remains relatively
stable even with temperature changes, and they can synchronize with environmental cues
like sunlight. Biological clocks are deeply involved in regulating our health,
and they are receiving attention in the medical field as well.
Now let's introduce our main character today, cyanobacteria. Cyanobacteria are bacteria
that perform oxygen-producing photosynthesis. For billions of years, they have supported
Earth's ecosystem. They are truly ancient pioneers of our planet. Photosynthesis is the process of
using light energy from the sun to convert carbon dioxide and water into organic matter and oxygen.
This process primarily happens during the day. But think about it, maintaining the system
シアノバクテリアの生物時計
necessary for photosynthesis during the night, when there's no sun, is a waste of energy.
It's like leaving the light on all night. So for cyanobacteria, the ability to predict
when the sun will rise and when it will set. In other words, having a biological clock
becomes very important. If they know when the sun rises, they can start preparing for
photosynthesis efficiently. Previous research has shown that the biological clock in certain
modern cyanobacteria is composed of just three proteins, Kai A, Kai B, and Kai C.
These are called clock proteins. What's amazing is that even if you take these three proteins
out of the cells and mix them in a test tube, they still exhibit a reasonable reaction with
a period of about 24 hours. However, how and when photosynthesizing bacteria actually acquired
this system for predicting time, this biological clock has remained a mystery in the history of
life for a long time. So the research group set out to tackle this mystery. Their approach is very
unique and exciting. And they decided to recreate the approximately three billion years evolutionary
process of the Kai clock proteins in the lab. How is it even possible? First, the research
group used amino acid sequence data for Kai proteins found in currently living cyanobacteria
and other bacteria. Amino acid sequence is like the blueprint of a protein. This blueprint
changes the diversity phase gradually throughout the course of evolution, spanning from ancient
times to the present. Using computers, the researchers reverse calculated these changes
in the amino acid sequence. This technique is called an ancestral sequence reconstruction.
It's like tracing a phylogenic tree, which is essentially a family tree of life, to estimate
the protein blueprint that a common ancestor might have possessed. So it's said to be similar
古代の細胞時計の再現
to how researchers and ancient document studies tried to reconstruct the original manuscript
from existing copies. In this way, they estimated the amino acid sequence of Kai proteins that
would have represented ancestral cyanobacteria from different eras, such as about 3.1 billion
years ago, and 2.6 billion years ago, about 2.2 billion years ago, about 1.3 billion years ago,
and about 100 million years ago. And they actually synthesized these ancestral Kai A,
Kai B, and Kai C proteins and recreated the ancestral biological clock in a test tube.
It's like bringing back components of ancient life using a time machine.
Next, they tested whether the ancestral biological clocks from each era could actually tick,
meaning they would show a rhythmic reaction. The result revealed that the clock proteins from
approximately 2.2 billion years ago were indeed the very first biological clocks in history
that clearly started ticking. There was a period between about 2.6 billion and 2.2 billion years
ago when the ability to keep rhythm involved significantly. So what kind of rhythms did this
first biological clock in Earth's history keep? And was it a 24-hour cycle like modern cyanobacteria
or our own biological clocks? Surprisingly, the rhythms kept by the oldest biological clock
dating back about 2.2 billion years ago was a cycle of 18 to 20 hours. This is incredibly exciting
because this period around 2.2 billion years ago is called the Paleopteroterozoic era,
and a geological sample analysis like the fossils has suggested that the Earth's rotation was faster
back then, with a day lasting around 20 hours. The results of this experiment match these
fossil analysis results perfectly. This means that the trace of Earth's rotation speed change
was clearly preserved within the evolutionary process of the cyanobacteria's biological clock.
生物時計の起源
It's evidence that life adjusted its biological clock to adapt to the environmental changes.
Furthermore, it's intriguing that this period around 2.2 billion years ago
coincides with a crucial event in Earth's history called the Great Oxidation Event.
The Great Oxidation Event was a time when the oxidation concentration in the atmosphere
sharply increased due to the photosynthesis by cyanobacteria. The acquisition of this biological
clock at this specific time suggested that it gradually contributed to the efficiency of
photosynthesis, which led to the Great Oxidation Event. It's as if cyanobacteria acquired a new
tool for living more efficiently, precisely when the Earth's environment was undergoing a dramatic
change. This study also investigated how this biological clock maintained such an accurate
rhythm. It was found that the secret to generate the rhythm lies in the molecular structure of the
proteins called Kaishi. Kaishi is driven by the breakdown of ATP, often called the energy currency
of life. This mechanism was supported by the fact that the arrangement of atoms within the Kaishi
protein had been maintained with an astonishing precision of 0.1 nanometers. The clockwork was
ticking with atomic level precision. This means that cyanobacteria, using a biological clock
maintained with atomic level precision, have been able to read the time from the rhythm and
constantly know when the sun rises and when the sun sets. At this time, management ability was
essential for efficiently operating systems like photosynthesis, which were involved in energy
acquisition and utilization. In order to survive the turbulent history on Earth since about
2.2 billion years ago, including dramatic environmental changes like global glaciation
and the rapid atmospheric composition shifts. To summarize this wonderful research,
シアノバクテリアの生物学的時計
that cyanobacteria acquired the oldest biological clock in the history of life, approximately 2.2
billion years ago. This biological clock ran a cycle of about 18 to 20 hours, matching the length
of the day on Earth at that time. The emergence of the biological clock coincided with a great
oxidation event, a period of rapid oxygen increase that dramatically changed Earth's
environment, suggesting it contributed to the efficiency of photosynthesis.
And this accurate rhythm was supported by the precisely maintained structure of the KSH protein
at the atomic level. This unveils a part of the ground survival strategy that cyanobacteria have
employed to adapt to Earth's environmental changes and survive for astonishing 2.2 billion years.
It's a study that reminds us how crucial time management is for life.
And so furthermore, this 2.2 billion years time management strategy of cyanobacteria suggests
that accurately knowing the time is incredibly important in modern fields like energy issues,
ecosystem maintenance, and even space exploration. Perhaps we can learn from life's time strategies
for future technologies and explorations. So if you are interested in this research,
please check it out in the description box. And SciencePod is delivered every weekday morning
in both Japanese and English. So I'd love for you to share your notes while listening to the
broadcast using the hashtag SciencePod. So see you next time.
