Genes Promoting Nerve, Other Cell Communications May Have Come From Bacteria
Some of the genes that allow nerve cells and some other types of
cells to send elaborate chemical messages to each other appear to
have been transferred to animals or their immediate ancestors from
bacteria eons ago, according to a study by researchers from the
National Library of Medicine and the National Institute of Child
Health and Human Development, both part of the National Institutes
Specifically, the genes contain the information needed to make
enzymes, which, in turn are crucial for making the complex molecules
that cells use to communicate with each other. These cell-signaling
molecules play a role in learning, memory, mental alertness, sleep
patterns, and allergic responses.
The study was published on the web at http://www.sciencedirect.com/science/journal/01689525
and will appear in the July issue of Trends in Genetics.
"By studying these enzymes in bacteria, we may be able to
get a better idea of how they work in human beings," said the
study's first author, Lakshminarayan Iyer, Ph.D., Research Fellow,
of the National Center for Biotechnology Information of the National
Library of Medicine (NLM).
Bacteria are single celled organisms. In plants and animals, DNA
is contained in a membrane bound compartment called the nucleus.
The DNA of bacteria is not contained within a nucleus.
For the study, the researchers conducted a comprehensive search
of the National Library of Medicine's genetic databases. They identified
a group of genes needed to make some enzymes involved in the manufacture
of the chemical messengers that cells use to communicate. The genes
are present in bacteria and in vertebrate animals, but with a few
exceptions, not in plants, or other complex living organisms. The
search was prompted by the group's earlier observation that the
enzyme arylalkylamine N-acetyltransferase (AANAT) was present in
animals, bacteria, and yeast, but in no other living organisms.
AANAT is used to make melatonin, a hormone that regulates the body's
cycles of sleeping and waking.
The researchers also identified genes for enzymes that are involved
in the manufacture of the following chemical messengers:
- acetylcholine involved in learning and memory, muscle contraction,
- dopamine the absence of which results in Parkinson's disease
- norepinephrine and epinephrine involved in alertness, vascular
- serotonin involved in mood,
- glutamate involved in alertness
- nitric oxide involved in many bodily functions, including
blood pressure regulation
- histamine involved in the allergic response
The bacterial genes may have been transferred to the organisms
that were the ancestors of animals more than a half billion years
ago, explained another of the study's authors, David Klein, Ph.D.,
a melatonin researcher at the National Institute of Child Health
and Human Development (NICHD).
It is not known how the genes were transferred, but Dr. Klein theorizes
that one form of transfer took place during the reproductive cycle,
with the genes having been incorporated into either sperm or egg
cells or incorporated shortly after fertilization. It's possible
that the transfer could also represent a form of infection where
genetic material is transferred into these reproductive cells and
thereby into the entire genome of the recipient.
Bacteria do transfer genes to other bacteria, by means of a circular
DNA molecule known as a plasmid. However, Dr. Klein said, bacteria
are not believed to be capable of passing plasmids to animal cells.
The study's authors offered an alternative explanation for the
fact that some genes are present only in bacteria and animals. According
to this explanation, all living organisms once possessed these genes
as well, and most lost them. However, the authors wrote that it
is unlikely that such a large group of living organisms could have
lost so many genes.
An understanding of how the enzymes function in bacteria may provide
insight into how they function in animals, Dr. Klein said. All the
enzymes may be important to bacteria because they provide a detoxification
function they make chemical changes within the bacteria that eliminate
potentially toxic substances. AANAT, he said, is present in both
the pineal gland, located in the brain, and in the retina of human
beings and other primates. In the pineal gland, AANAT plays a role
in manufacturing melatonin. However, AANAT in the retina does not
manufacture melatonin. Dr. Klein suspects that, in the retina, AANAT
may have a role in neutralizing and eliminating toxic substances.
He is currently investigating whether a disruption in AANAT function
plays a role in the development of macular degeneration, a disease
that impairs vision and that may result in blindness.
The NICHD is part of the National Institutes of Health (NIH),
the biomedical research arm of the federal government. NIH is an
agency of the U.S. Department of Health and Human Services. The
NICHD sponsors research on development, before and after birth;
maternal, child, and family health; reproductive biology and population
issues; and medical rehabilitation. NICHD publications, as well
as information about the Institute, are available from the NICHD
Web site, http://www.nichd.nih.gov,
or from the NICHD Information Resource Center, 1-800-370-2943; e-mail