Summary: Wild‐type Anabaena sp. strain PCC 7120, a filamentous nitrogen‐fixing cyanobacterium, produces single heterocysts at semi‐regular intervals. asr0100 (patU5) and alr0101 (patU3) are homologous to the 5′ and 3′ portions of patU of Nostoc punctiforme. alr0099 (hetZ) overlaps the 5′ end of patU5. hetZ, patU5 and patU3 were all upregulated, or expressed specifically, in proheterocysts and heterocysts. Mutants of hetZ showed delayed or no heterocyst differentiation. In contrast, a patU3 mutation produced a multiple contiguous heterocyst (Mch) phenotype and restored the formation of otherwise lost intercalary heterocysts in a patA background. Decreasing the expression of patU3 greatly increased the frequency of heterocysts in a mini‐patS strain. Two promoter regions and two principal, corresponding transcripts were detected in the hetZ‐patU5‐patU3 region. Transcription of hetZ was upregulated in a hetZ mutant and downregulated in a patU3 mutant. When mutants hetZ::C.K2 and hetZ::Tn5‐1087b were nitrogen‐deprived, P_hetC‐gfp was very weakly expressed, and in hetZ::Tn5‐1087b, P_hetR‐gfp was relatively strongly expressed in cells that had neither a regular pattern nor altered morphology. We conclude that the hetZ‐patU5‐patU3 cluster plays an important role in co‐ordination of heterocyst differentiation and pattern formation. The presence of homologous clusters in filamentous genera without heterocysts is suggestive of a more general role.

Glycogen phosphorylase (GlgP, EC 2.4.1.1) catalyzes the cleavage of glycogen into glucose-1-phosphate (Glc-1-P), the first step in glycogen catabolis

Unlike those of the wild-type strain, proheterocysts of the Anabaena sp. strain PCC 7120 hetC strain keep dividing. ftsZ , the most critical cell division gene, is up-regulated in hetC proheterocysts. Heterocyst differentiation genes hglD , hglE , patB , nifB , and xisA are no longer expressed in the hetC mutant. hetC also regulates the expression of patA , a pattern formation gene.

In adaptation to new environments, organisms may accumulate mutations within encoding sequences to modify protein characteristics or acquire mutations within regulatory sequences to alter gene expression levels. With the development of antifreeze capability as the example, this study presents the evidence that change in gene expression level is probably the most important mechanism for adaptive evolution in a green alga Chlorella vulgaris. C. vulgaris NJ-7, an isolate from Antarctica, possesses an 18S rRNA sequence identical to that of a temperate isolate, SAG211-11b/UTEX259, but shows much higher freeze tolerance than the later isolate. The chromosomal DNA/cDNA of four antifreeze genes, namely hiC6, hiC12, rpl10a and hsp70, from the two isolates of C. vulgaris were cloned and sequenced, and very few variations of deduced amino acid sequences were found. In contrast, the transcription of hiC6, hiC12 and rpl10a was greatly intensified in NJ-7 compared to that in UTEX259, which is correlated to the significantly enhanced freeze tolerance of the Antarctica isolate.

Unlike Escherichia coli , the cyanobacterium Synechocystis sp. strain PCC 6803 is insensitive to chill (5°C) in the dark but rapidly losses viability when exposed to chill in the light (100 μmol photons m ⁻² s ⁻¹ ). Preconditioning at a low temperature (15°C) greatly enhances the chill-light tolerance of Synechocystis sp. strain PCC 6803. This phenomenon is called acquired chill-light tolerance (ACLT). Preconditioned wild-type cells maintained a substantially higher level of α-tocopherol after exposure to chill-light stress. Mutants unable to synthesize α-tocopherol, such as slr1736, slr1737, slr0089, and slr0090 mutants, almost completely lost ACLT. When exposed to chill without light, these mutants showed no or a slight difference from the wild type. When complemented, the slr0089 mutant regained its ACLT. Copper-regulated expression of slr0090 from P _petE controlled the level of α-tocopherol and ACLT. We conclude that α-tocopherol is essential for ACLT of Synechocystis sp. strain PCC 6803. The role of α-tocopherol in ACLT may be based largely on a nonantioxidant activity that is not possessed by other tocopherols or pathway intermediates.