STRESS RESPONSES - Cyanobacteria - page 650

(1998) speculated that the CSDs are ancient structures that existed even before the divergence of
prokaryotes and eukaryotes. It is interesting to note that cyanobacteria do not possess CSDs but
instead possess Rbps (Sato, 1995) that also possess a three-dimensional functional surface required
for nucleic acid binding (Graumann and Marahiel, 1996). During the course of evolution, Rbps may
have replaced CSDs in cyanobacteria (Graumann and Marahiel, 1998).
a) RNA-binding proteins : A gene ( rbpA ) that encodes an Rbp was for the fi rst time reported by Sato
(1994) in A . variabilis M3. This gene has been subsequently redesignated as rbpA1 (Sato, 1995) and was
shown to be organized in an operon with a ribosomal protein gene rpsU . A temperature shift-down
from 38 to 22°C caused a drastic increase in the level of the transcript of rbpA1 and it disappeared
rapidly after a shift-up from 22°C to 38°C. The RbpA protein (it may be noted that the protein name
Rbp is derived from the gene name ' rbp ' and it is not an abbreviation for 'RNA-binding protein'
hence we can say Rbp protein) that was expressed in E . coli exhibited a strong affi nity with RNA
isolated from Anabaena cells. The RbpA protein contained only one RNA-recognition motif (RRM)
with a short C-terminal glycine-rich sequence. This constitutes one of the smallest among known
Rbps. Other cyanobacteria which are reported to possess Rbp genes are S. elongatus PCC 6301 (Sugita
and Sugiura, 1994), Chlorogloeopsis sp. PCC 6912, Anabaena sp. strain PCC 7120 (Mulligan et al ., 1994),
Synechocystis sp. strain PCC 6803 (Kaneko et al. , 1996), S. elongatus PCC 7942 and Synechococcus sp.
strain PCC 7002 (Sato et al. , 1997a). In cyanobacteria there appears to be a correlation between the
copy number of rbp genes and the size of the genome of the organism. S. elongatus PCC 6301 with
a 2.6 Mb genome possesses two copies of rbp genes. As the genome size of Synechocystis sp. strain
PCC 6803 is 3.6 Mb the number of copies of rbp genes increased to three (Kaneko et al ., 1996). In
A . variabils M3, Anabaena sp. strain PCC 7118 and Anabaena sp. strain PCC 7120 with an estimated
genome size of 6.4 Mb in each case, the number of rbp genes increased to eight (Bancroft et al ., 1989).
Since the copy numbers of rbp genes seem to be consistent with the increase in genome size, the
increase in copy number of rbp genes could have resulted due to duplication of the whole genome
and not by simple gene duplication events. These genes encode small Rbps with one RRM with a
short C-terminal glycine rich sequence bearing resemblance to the RbpA initially reported by Sato
(1994). The Rbps seem to be unique for cyanobacteria as among the prokaryotes Helicobacter pylori
(Tomb et al ., 1997) and Treponema pallidum (Fraser et al ., 1998) are the only other bacteria known to
possess the rbp genes. It is interesting to note that the cyanobacterial RRM sequences are very much
similar to chloroplast RRMs which supports the endosymbiotic origin of chloroplast (Sugita and
Sugiura, 1994; Mulligan et al ., 1994).
Southern blot and immunoblot analyses revealed the existence of at least eight genes in
A . variabilis M3 that encode Rbp proteins. Of the eight, the cloning of fi ve of them ( rbpA1 , rbpA3 ,
rbpB , rbpC and rbpD ) was initially reported (Sato, 1994, 1995; Sato and Maruyama, 1997). The rest of
the three genes ( rbpA2 , rbpE and rbpF ) have been cloned subsequently (Maruyama et al ., 1999). Seven
of the A . variabilis M3 Rbps possess a single RRM with a short C-terminal glycine rich sequence but
RbpD protein lacks such a C-terminal glycine rich sequence. Moreover, exceptionally RbpD protein
only contains a probable cAMP- and cGMP-dependent protein kinase phosphorylation site in the
C-terminal region. Although Sato (1995) reported that the nucleotide sequence of rbpA2 gene in
A . variabilis M3 appeared to be identical to that of the rbpA gene of Anabaena sp. strain PCC 7120,
Maruyama et al . (1999) observed small changes between the nucleotide positions 55, 299 and 300
from the initiation codon. These differences contributed to the variation in the sequences of amino
acids at the corresponding positions. The amino acid sequences of these gene products were highly
conserved, except that the RbpD protein lacked glycine-rich C-terminal domain present in all other
known members of the gene family.