Characterization of a blue-green algal genome

Transformation in Agmenellum quadruplicatum

A DNA-mediated transformation system for the blue-green alga Agmenellum quadruplicatum, strain PR-6, is described and characterized for DNA concentration dependence, dependence on time of exposure to DNA, phenotypic expression, sensitivity to various enzymes, and competence. The stability of the transformants has been investigated, and genetic backcross and selfing experiments have been performed. This system fulfills all of the criteria established for the well-characterized transformation systems in heterotrophic bacteria and demonstrates significant similarities to at least one of these systems for all characteristics examined. The efficiency of transformation is high. This system fills a need for a well-characterized genetic system in an oxygen-evolving photoautotroph. We have used it to transform a strain with a mutational lesion in assimilatory nitrogen metabolism to a wild-type genotype.

Development and field testing of a real-time PCR assay for cylindrospermopsin-producing cyanobacteria

AIMS

To develop and test a real-time PCR assay to detect and quantify genes specific to Cylindrospermopsis sp. and cylindrospermopsin-producing cyanobacteria.

METHOD AND RESULTS

A duplex real-time PCR assay was developed that targets a cylindrospermopsin-specific and Cylindrospermopsis raciborskii-specific DNA sequence. The C. raciborskii-specific sequence was based on the rpoC1 DNA-dependent RNA polymerase gene, whilst the cylindrospermopsin-specific sequence was selected by surveying an extensive number of potential cylindrospermopsin-producing cyanobacterial strains for genes implicated in toxin production, aoaA, aoaB and aoaC. In toxic strains, sequences of each of these three genes were always present; whilst in nontoxic strains the distribution of these sequences was patchy, resulting in what are likely to be natural deletion mutants. The real-time assay was optimized on a fixed and portable device, with results indicating that the reliable limit of detection for the assay was 100 copies per reaction or 1000 cells ml(-1) for both target sequences on both devices. In routine environmental samples enumerated by microscopy, the assay results were positive for all samples where C. raciborskii cells were observed at >1000 cells ml(-1) and negative in 15 samples where no C. raciborskii cells were observed. In field samples, the number of copies of the rpoC1 sequence more closely approximated the number of cells enumerated by microscopy, the number of copies of the pks sequence and detection of the toxin-specific sequence matched the results of toxin testing.

CONCLUSIONS

The duplex real-time PCR assay was a sensitive and rapid method for detecting potential cylindrospermopsin-producing cyanobacteria in the laboratory or in the field. The observation of probable natural deletion mutants provides further evidence that the aoaA, aoaB and aoaC genes are involved in toxin production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This assay provides a new monitoring capability for tracking cylindrospermopsin-producing cyanobacteria that are an emerging threat to water quality.

Salt stress induces programmed cell death in prokaryotic organism Anabaena

AIMS

Our main interest is to check if programmed cell death (PCD) can occur in prokaryotic algae and if the morphological and biochemical features of PCD are conserved.

METHODS AND RESULTS

Using TUNEL labelling, fluorescence and light microscopy and DNA gel electrophoresis, we found that cell death with features similar to those in metazoan PCD could be induced in different Anabaena strains after exposure to univalent-cation salts at moderate concentration. These features included specific DNA fragmentation, cytoplasmic vacuolation, and the progressive disorganization, fragmentation and subsequent autolysis of the cell corpse. Further analyses of cell viability and proteinase activity revealed that increased protease activities, decreased DNA content, and loss of plasmalemma integrity were related to the PCD process.

CONCLUSIONS

The results showed that like PCD in eukaryotes, PCD in Anabaena is an active process, and is an adaptation to adverse environments. The features of PCD shared between eukaryotes and Anabaena suggest that PCD mechanisms are conserved during evolution.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results will contribute greatly to our understanding of PCD origin and evolution, and are potentially useful in controlling the deluge of algae in some lakes.

Cell Cycle Regulation in Marine Synechococcus sp. Strains

The cell cycle behavior of four marine strains of the unicellular cyanobacterium Synechococcus sp. was analyzed by examining the DNA frequency distributions of exponentially growing and dark-blocked populations and by considering the patterns of change in these distributions during growth under a diel light-dark cycle. The two modes of cell cycle regulation previously identified in a freshwater and coastal marine Synechococcus isolate, respectively, were represented among the three open-ocean strains we examined. The first of these modes of regulation is consistent with the slow-growth case of the widely accepted prokaryotic cell cycle paradigm. The second appears to involve asynchronous initiation of chromosome replication, the presence of multiple chromosome copies at low growth rates, and variability in chromosome copy number among cells in the population. These characteristics suggest the involvement of a large probabilistic component in cell cycle regulation which could make the application of cell cycle-based estimators of in situ growth rate to Synechococcus populations problematic.

Significance and measurement of DNA double strand breaks in mammalian cells

Techniques for measuring DNA double strand breaks in mammalian cells are being used increasingly by researchers studying both physiological processes, such as recombination, replication, and apoptosis, as well as pathological processes, such as clastogenesis induced by ionizing radiation, chemotherapeutic drugs, and chemical toxicants. In this review we evaluate commonly used assays for measuring DNA double strand breaks, focusing on neutral filter elution and pulsed field gel electrophoresis, and explore the advantages and limitations of applying these techniques to problems of current interest in carcinogenesis and genetic toxicology.

Relationship between DNA cycle and growth rate in Synechococcus sp. strain PCC 6301

Flow cytometry was used to examine cell cycle regulation in Synechococcus sp. strain PCC 6301 under a variety of growth conditions. The DNA frequency distributions of exponentially growing and dark-blocked populations confirmed that this cyanobacterium contains multiple chromosome copies even at very slow growth rates. Furthermore, the presence of major peaks corresponding to other than 2" chromosome copies strongly suggests that DNA replication is initiated asynchronously. Although this suggestion is at odds with the standard formulation of the procaryotic cell cycle model, it is similar to recent observations of asynchrony in Escherichia coli replication mutants.

Effect of Light on the Cell Cycle of a Marine Synechococcus Strain

Light-dependent regulation of cell cycle progression in the marine cyanobacterium Synechococcus strain WH-8101 was demonstrated through the use of flow cytometry. Our results show that, similar to eucaryotic cells, marine Synechococcus spp. display two gaps in DNA synthesis, at the beginning and at the end of the cell cycle. Progression through each of these gaps requires light, and their durations lengthen under light limitation.

Genes encoding core components of the phycobilisome in the cyanobacterium Calothrix sp. strain PCC 7601: occurrence of a multigene family

The phycobilisome is the major light-harvesting complex of cyanobacteria. It is composed of a central core from which six rods radiate. Allphycocyanin, an alpha beta oligomer (alpha AP and beta AP), is the main component of the core which also contains three other phycobiliproteins (alpha APB, beta 18.3, and L92CM) and a small linker polypeptide (L7.8C). By heterologous DNA hybridization, two EcoRI DNA fragments of 3.5 and 3.7 kilobases have been cloned from the chromatically adapting cyanobacterium Calothrix sp. strain PCC 7601. Nucleotide sequence determination has allowed the identification of five apc genes: apcA1 (alpha AP1), apcA2 (alpha AP2), apcB1 (beta AP1), apcC (L7.8C), and apcE (L92CM). Four of these genes are adjacent on the chromosome and form the apcEA1B1C gene cluster. In contrast, no genes have been found close to the apcA2 gene which is carried by the 3.5-kilobase EcoRI fragment. Transcriptional analysis and 5'-end-mapping experiments were performed. The results obtained demonstrate that the apcEA1B1C gene cluster forms an operon from which segmented transcripts originate, whereas the apcA2 gene behaves as a monocistronic unit. Qualitatively, the same transcripts were identified regardless of the light wavelengths received during cell growth. The deduced amino acid sequences of the apc gene products are very similar to their known homologs of either cyanobacterial or eucaryotic origin. It was interesting, however, that in the apcA1 and apcA2 genes, whose products correspond to alpha-type allophycocyanin subunits, nucleotide sequences were more conserved (67%) than were the deduced amino acid sequences (59%).

Critical DNA target size model of ionizing radiation-induced mammalian cell death

A new model of mammalian cell killing by ionizing radiation is presented. This model, termed the critical DNA target size model, postulates that DNA double-strand breakage is the critical radiation-induced lesion and that the dose-response for such breakage can be non-linear due to the action of a saturable chemical repair process. DNA double-strand breakage occurring within critical targets (proto-oncogene- or common fragile site-associated sequences) is postulated to initiate recombination events with undamaged sequences, leading to chromosomal aberrations. The subsequent loss of acentric fragments at mitosis is postulated to prevent the continuity of the genome and to produce cell death by the induction of chromatin structural changes. Experimental evidence contrary to other radiation action models is examined, and the hypotheses of the model are justified.

The dose-response for low-LET radiation-induced DNA double-strand breakage: methods of measurement and implications for radiation action models

There is considerable controversy over the form of the dose-response for DNA double-strand breakage (dsb) induction in mammalian cells by low-LET type radiation. This controversy centres on the techniques used for measuring DNA dsb. The applications and shortcomings of the four major techniques for estimating DNA size--sedimentation, viscoelastometry, electrophoresis, and non-denaturing filter elution--are examined. In particular, the criticisms of the results obtained using the non-denaturing filter elution technique, which have suggested that the DNA dsb dose-response is non-linear, are discussed. It is concluded that these results may require a re-evaluation of the basic assumptions of many radiation action models.

Molecular weight of T2 NaDNA from viscoelasticity

The viscoelastic properties of T2 DNA solutions are used to determine the NaDNA molecular weight in four independent ways from the theory of the beads-springs model. The four molecular weights are 131.9, 132.7, 130.5, and 127.6 X 10(6). The average of these values, adjusted for the probable errors in viscoelasticity and concentration measurements, is (126 +/- 5) X 10(6). The four molecular weights are termed Mtaugamma11, Mtaueta, Mtaugamma, and MtauA; each is different in its sensitivity to molecular weight distribution. Their agreement suggests (1) that the theoretical equations relating each M to the corresponding measured properties are valid, (2) that T2 DNA behaves as a partially free-draining polymer chain, and (3) that our solutions were nearly homogeneous in DNA size. We show that serious errors can result if the viscoelastic properties are not extrapolated to their limits at zero shear rate, as well as at zero DNA concentration, before calculating molecular weight.