MOVEMENTS - Cyanobacteria - page 290

is greatly affected. All these genes in turn appear to be controlled by the cAMP-receptor protein
(CRP) of Synechocystis sp. strain PCC 6803, SYCRP1 (Dienst et al ., 2008).
CRPs are global transcriptional regulatory proteins that bind to sequence-specifi c promoter
regions of the genes under their control. Hedger et al . (2009) identifi ed additional target genes for
SYCRP1 of Synechocystis sp. strain PCC 6803 and the transcription of these genes, i.e. slr1351 (mufF),
sll1874 (ChlA II ), sll1708 (NarL) and slr0442 is stimulated by illumination. According to them the
binding of SYCRP1 to the promoter regions is similar to the binding of CRPs to Class I and Class
II promoter regions of E . coli and so a similar mechanism of transcriptional activation is envisaged.
P. aeruginosa is the best studied system to understand twitching motility mediated by Tfp. Genetic
studies have established that there are at least 40 genes involved (McBride, 2001). The genes concerned
with pilus biogenesis are designated as pil genes. Gene products PilU and PilT1 provide energy
for retraction of Tfp while PilB provides energy essential for assembly of pilin sub-units. The pilin
protein PilA1 which forms the basic structure of Tfp is synthesized by pilA1 gene. PilD is required for
processing of pre-pilin which serves as a bifunctional enzyme for N-terminal processing of prepilin
and for methylation of N-terminal amino acid of the mature protein. PilE, PilV, FimT and FimU are
other pre-pilin like proteins which are also processed by PilD. The functions of these prepilins are
not known. PilQ is required to allow Tfp to cross the outer membrane. This protein forms highly
stable complexes of 12-14 subunits with central channels that range from 5 to 10 nm in diameter.
i) cAMP and twitching motility : cAMP is known as an important signalling molecule in prokaryotes
as well as eukaryotes. Cellular cAMP levels are known to change in response to changes in
environmental conditions such as light-dark, low pH-high pH and oxic-anoxic states (Ohmori et al .,
1988; Ohmori, 1989) and in case of nitrogen defi ciency and suffi ciency (Hood et al ., 1979). The addition
of cAMP to cells of Spirulina platensis stimulated respiration and gliding movement (Ohmori et al .,
1992). The biosynthesis of cAMP is mediated by the enzyme adenylcyclase (Cya). Of the six classes
of Cyas known, only Class III type is the universal class widely distributed among prokaryotes and
eukaryotes. Terauchi and Ohmori (1999) identifi ed two genes, Cya1 ( slr1991 ) and Cya2 (sll0646) that
encode Cyas in Synechocystis sp. strain PCC 6803. These bear homology to the eukaryotic Cyas and
belong to the Class III . The disruption of Cya1 gene resulted in a simultaneous reduction in cAMP
levels with simultaneous loss of motility. The addition of extracellular cAMP regained motility of
Synechocystis sp. strain PCC 6803. SYCRP1 binds to the promoter regions of genes under its control
in presence of cAMP in Synechocystis sp. strain PCC 6803 (Yoshimura et al ., 2000). Disruption of the
gene for cAMP receptor protein (SYCRP1 , sll1371) in Synechocystis sp. strain PCC 6803 resulted in
mutants that are non-motile with reduced Tfp on the cell surface signifying that cAMP-SYCRP1
complex controls the biogenesis of Tfp (Yoshimura et al ., 2002). On the contrary, a re-examination
of the motility behaviour of the cya1 and sycrp1 gene disruptant mutants of Synechocystis sp. strain
PCC 6803 suggested that the mutants exhibited motility and phototactic behavior. But these mutants
seem to be impaired in one particular phase of phototaxis (Bhaya et al ., 2006). Blue light (450
nm) markedly increased cAMP content in the cells of Synechocystis sp. strain PCC 6803 while red
(630 nm) and far red (720 nm) lights did not. Disruption of cya1 gene encoding adenylycyclase resulted
in no appreciable effect on cellular cAMP level even in blue light. Accordingly, blue light stimulated
motility in wild-type while in cya1 mutant cells blue light did not stimulate motility (Terauchi and
Ohmori, 2004). They thus concluded that a blue light-cAMP signal transduction system is operative
in the motility of Synechocystis sp. strain PCC 6803.
ii) pil Genes: Genome sequencing of Synechocystis sp. strain PCC 6803 (Kaneko et al ., 1996) facilitated
the identifi cation of a number of genes involved in motility and phototaxis. By undertaking