HETEROCYST - Cyanobacteria - page 197

Het - phenotype now produced only 2-8% proheterocysts without shaking the cultures but upon
shaking the cultures it neither produced heterocysts nor a pattern of phycobilisome fl uorescence.
Likewise, in a second mutant DRHB2.14 (in which the Tn903 got inserted at 615 bp from 5'-end
of ORF alr0099 ) reported by Ning and Xu (2004b) and in the mutant hetZ :: C.K2 the differentiation
of heterocysts proceeded with a normal pattern after nitrogen step-down but mature heterocysts
appeared less than 1%. Similarly, FQ422 mutant (with insertion of transposon at 58 bp upstream of
the 3' end of the ORF alr0099 ) described by Fan et al . (2005) also had a similar phenotype. Because
of the essential nature of alr0099 for heterocyst differentiation it was designated as hetZ by Zhang et
al . (2007). The up-regulation of hetZ , patU5 and patU3 was noted in proheterocysts and heterocysts.
A mutant of patU3 showed Mch-phenotype but when introduced into a patA - mutant it restored the
formation of intercalary heterocysts. The phenotype of patU3 mutant resembles the phenotype of a
patS deletion mutant. But PatU3 does not resemble either PatS (Yoon and Golden, 1998) or for that
matter HetN (Li et al ., 2002) in having the pentapeptide RGSGR. A very weak expression pattern
of hetC in mutants hetZ :: C.K.2 and hetZ::Tn5-10876 and a very strong expression of hetR in the cells
of the latter mutant without any regular pattern are suggestive of an important role for the gene
cluster hetZ - patU5 - patU3 .
ix) asr1734 : Gene asr1734 of Anabaena sp. strain PCC 7120 encodes a protein Asr1734 of 93 amino
acid residues that negatively regulates heterocyst differentiation Wu et al. (2007) . Asr1734 inhibited
heterocyst formation when present in extra copies or when it is overexpressed in wild-type or in
strains AMC451 (mutant of patS with asr1734 overexpressed) and AMC1286 (mutant of hetR Arg229Trp
with asr1734 overexpressed) that exhibited Mch-phenotype. But in the wild-type overexpression of
asr1734 brought about degradation of phycobiliproteins in the vegetative cells. The expression of
P asr1734 - gfp as a reporter in the wild-type after a nitrogen step-down was restricted to the developing
proheterocysts or heterocysts. Deletion of asr1734 from the chromosome by the introduction of a
suicide plasmid pAM3234 by homologous recombination resulted in a strain designated as AMC1252.
This strain showed a constitutive formation of heterocysts (2%) in nitrate medium but differentiated
high frequency of heterocysts (15%) after a nitrogen step-down. Ammonium grown cultures of
AMC1252 when shifted to nitrate medium produced 5% heterocysts. Transcripts for ntcA and hetR
increased in AMC1252 and in an asr1734 overexpression strain after 6 h of nitrogen step-down. It
is thus concluded that Asr1734 inhibits heterocyst differentiation downstream of hetR . Asr1734 has
been predicted to have α-helical structure and the C-terminal region contains several basic amino
acids. Mutation of Arg84 and Arg86 resulted in a double mutant Arg84Gly/Arg86Gly and such
mutated asr1734 when introduced into wild-type there was no inhibition of heterocyst differentiation
showing there by the requirement of C-terminal region for stability or function of Asr1734 Wu et al.
(2007). It is to be noted that homologues of asr1734 are not present in unicellular cyanobacteria or
the non-heterocystous fi lamentous T . erythraeum but database searches revealed its orthologues to
be present in only heterocystous forms like A . variabilis ATCC 29413 and N . punctiforme PCC 73102.
However, in N . punctiforme PCC 73102 it is considered to be an orphan gene and the predicted crystal
structure of the gene product Npun_R1517 revealed an extensively interlocked homodimer structure
with a number of clefts and cavities (Ni et al ., 2009).
B) Late genes
i) Development genes : The genes that are expressed considerably at late stages (around 12 to 14 h
after nitrogen step-down) of heterocyst development and maturation are classifi ed as development