In a study that could lead to new ways to prevent infection by human
immunodeficiency virus (HIV) and similar organisms, Purdue University
researchers have been able to genetically modify a plant to halt
reproduction of a related virus.
Cauliflower mosaic virus attacks a group of plants that includes the
largest number of agriculturally important plants in the world. The plant
virus and HIV, which causes AIDS, use the same process to multiply in their
victims' cells and spread disease.

"After HIV infects a person, it must recruit and latch onto particular
human proteins so that the virus can replicate throughout the body," said
Zhixiang Chen, a Purdue professor of botany and plant pathology. "We found
that cauliflower mosaic virus relies on the same protein complex to multiply
in plants."

Cauliflower mosaic virus, known as CaMV, attacks a plant group that
includes cauliflower, broccoli, cabbages, turnips, canola and many types of
mustard.

"We believe that the proteins in these host plants might be
particularly important for these types of viruses, such as HIV, because if
you block them, then the viruses simply can't replicate."

The retrovirus HIV and the pararetrovirus CaMV both use reverse
transcription to recruit the host's proteins in order to reproduce and
spread infection. Transcription in cells is the process in which a gene's
DNA code is copied into RNA, which, in turn, carries the information to
another part of the cell or to another cell. In reverse transcription, used
by viruses such as HIV and CaMV, the virus' RNA is copied into DNA after it
latches on to a victim's cell. This allows the virus to easily integrate
into the host's genome and then reproduce in other cells.

Chen and his colleagues published a report on their study in the most
recent issue of the journal The Plant Cell.

The researchers found that in the laboratory research plant
Arabidopsis, cauliflower mosaic virus recruits a protein complex called
CDKC. This is the same protein complex that HIV uses, known in humans as
P-TEFb. Since both viruses use this same process to trigger transcription,
the scientists now know that this protein complex and its related genes have
passed from species to species as organisms evolved over millions of years,
Chen said.

"P-TEFb appears to be an evolutionarily conserved target of complex
retro- and pararetroviruses for activating transcription," he said. "This
must also reflect a fundamental mechanism for transcription inherited by
these viruses."

Humans and organisms used for research, such as fruit flies and the
tiny wormlike organism Caenorhabditis elegans, have only one gene in the
protein complex that retroviruses use to activate transcription. These
organisms die if that gene is completely blocked because of its essential
role during transcription. This makes it difficult to analyze the function
the gene may have in the organisms' growth, development and survival. Unlike
those other organisms, the plant protein complex involves two genes.

"In Arabidopsis there are two genes for the CDKC protein complexes
that trigger the transcription process," Chen said. "If we knock out one of
these genes, the plants become resistant to CaMV and the plant is still
growing."

The discovery of these two genes suggests that the mustard plant
Arabidopsis is a better organism than others for studying how the proteins
regulate gene function and transcription, he said.

However, blocking of one of the plant's genes caused some alteration
of leaves, flowers and trichomes (tiny hairlike structures) and delayed
flowering on the mutated plants, he said. In addition, mutant plants in
which both genes were blocked died in the embryonic stage just as would an
organism with only one gene.

Now that Chen knows that Arabidopsis has two genes involved in the
transcription process, his research team wants to learn more about genes'
possible roles in plant growth and development and where those tasks are
performed.

"The two genes each may have specialized functions depending on where
they are activated in the plant," he said. "In some tissues the genes appear
to be turned on in the same place. But, for example, in the flower, one gene
is expressed in one particular place and the other gene is expressed in a
different place."

The key question for researchers is how blocking the function of one
protein inhibits transcription and replication of the viruses. Discovering
the answer could mean major advances for prevention of retroviruses and
treatment of the diseases they cause in plants and animals.


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