Page 121 - Algae Anatomy, Biochemistry, and Biotechnology

104 Algae: Anatomy, Biochemistry, and Biotechnology

for using bovine rhodopsin opsin complementary DNA (cDNA) probe to identify homologous genes
in other species was demonstrated by Martin et al. (1986). These authors identified coding regions of
bovine opsin that are homologous with visual pigment genes of vertebrate, invertebrate, and photo-
tactic unicellular species. Successful application of this method requires closely homologous genes,
and in general additional criteria, such as protein sequence information, is desirable for eliminating
false positives on Southern blots. A molecular biology approach has been used also by Sineshchekov
et al. (2002) in Chlamydomonas. These authors identified gene fragments with homology to the
archaeal rhodopsin apoprotein genes in the expressed sequence-tag data bank of Chlamydomonas
reinhardtii. Two quite similar genes were identified having almost all the residues of bacteriorho-
dopsin in the retinal binding site. The authors suspected that these genes were related to the putative
retinal-based pigments already suggested for Chlamydomonas.
However, to show that the pigments are a part of the genuine signaling system, ideally one
would like to delete each gene by using homologous recombination, but it is not easy to do such
gene knockouts in any algal species. The problem can be overcome partially by using RNA inter-
ference (RNAi) technology to preferentially suppress the synthesis of the pigments to convincingly
show that the pigment is a genuine segment of the algal phototactic response.
Understanding the molecular mechanism used by algal cells to “see the light,” as we have tried
to explain, is a very difficult task. At least a century has been wasted without any success. It is dis-
couraging to think that even if the algae are not as intelligent as men are, they have “understood”
very well how to orientate themselves in their light environment, and do it very efficiently. Maybe
the compass mechanism they use is too simple for our complex brain.

HOW ALGAE USE LIGHT INFORMATION
No physical quantity regulates and stimulates the developments of algae as strongly as light. Light
is an electromagnetic radiation characterized by its quality (different wavelengths) and intensity. To
detect light and to measure both parameters and react to them, algae photoreceptor systems have to
satisfy five main requirements:

. They should possess a photocycling protein
. They should possess high sensitivity
. They should be characterized by a low noise level
. They should detect either spatial or temporal patterns of light
. They should transmit the detected signal in order to modify the cell behavior

Photocycling Proteins

Upon absorption of a photon, the photocycling protein undergoes a series of conformational
changes generating intermediate state(s); one of these states is the “active” state that will start
signal transmission. The last intermediate state is driven back to the original state of the protein,
by either a thermal process, or a second absorbed photon of different wavelength. The primary
event in the photoreceptive process is the structural change of the chromophore (isomerization)
to which the protein adapts. It occurs within a few picoseconds after the absorption of a photon,
and this is one of the fastest biological processes in nature. The whole photocycle is very fast
(order of microseconds or less), hence the intracellular response is immediately reset so that the
system is prepared for a new light signal, and algae must respond rapidly on a time scale of milli-
seconds to seconds as environmental conditions change or as they change position relative to their
static surroundings. A photoreceptor protein capable of photocycling is mandatory for algae whose
photoreceptive systems are an integral part of the cell body.
This localization would not allow the continuous recovery of the exhausted photoreceptive
proteins without interfering with a continuous and immediate response of the alga cell to the light.