Biology Reference
In-Depth Information
does not result in any general defect in transport of IFT or flagellar mem-
brane proteins; however, the
bbs4
strain does show accumulation of four
specific proteins within the flagella. Three of the proteins appear to be
membrane-bound and were identified as putative signalling proteins. The
fourth protein accumulates within the flagella is not membrane-bound
and was identified as the gene product of the
THB1
gene using mass spec-
trometry. The failure of
bbs4
to clear the THB1 protein results in its accu-
mulation in a process apparently leading to progressive loss of phototaxis.
This study, as well as a subsequent investigation (
Lechtreck et al., 2013
), sug-
gests that the THB1 protein is a component of a signalling pathway within
the flagella and it reinforces, along with the DCMU-dependent expression
of
LI637
(
Gagn
ยด
& Guertin, 1992
), a novel role for the TrHb proteins in
green algae.
An additional advantage in using
C. reinhardtii
is the genome-wide gene
expression analysis available for this species (
Grossman et al., 2003
). In a
screen of the effects of anoxia on gene regulation, the gene for a predicted
TrHb (
THB8
) showed over 1000-fold increase in expression when oxygen
was removed from the system (
Hemschemeier et al., 2013
). These
researchers also created a novel strain of
C. reinhardtii
where the transcript
level of the
THB8
gene was substantially reduced through artificial micro-
RNA silencing. The resulting strain showed greatly impaired growth under
low oxygen conditions and increased sensitivity to nitric oxide. The work
demonstrates the great value that a model organism such as
C. reinhardtii
plays in the functional studies of haemoglobins in algae.
Several additional
C. reinhardtii
reports, although not directly relating to
the topic of this chapter, provide supplemental information including data
on THBs in response to stress conditions. In one study (
Gonzalez-
Ballester et al., 2010
), the transcript levels of a mutant of
C. reinhardtii
impaired in sulphur accumulation are compared to those of wild-type cells
under both sulphur-replete and sulphur-deprived conditions. The transcript
level of
THB1
increases by several fold when sulphur is depleted from the
medium. The authors note that extreme sulphur deprivation caused cell
stress due to an increase in reactive oxygen formation within the chloroplast.
A second study (
Fischer et al., 2012
) investigated a mutant of
C. reinhardtii
with singlet oxygen resistance. The mutant in this chapter (
sor1
) causes the
up-regulation of genes related to detoxification using the glutathione
pathway. The THB proteins were not identified in this pathway, but the
genome-wide analysis does reveal that the expression of
THB1
increases
significantly when wild-type cells are exposed to chemicals that create
