Project description:Cyanobacteria Prochlorococcus marinus subsp. pastoris str. CCMP1986 (MED4) and Prochlorococcus marinus str. MIT 9313 (MIT9313) are oceanic oxygenic phototrophs, where MED4 is abundant in surface waters (~0-50 meters) and MIT9313 is abundant at depths of ~100 meters. To explore nitrogen-regulated changes in gene expression in these Prochlorococcus ecotypes, log phase cultures of MED4 and MIT9313 were transferred to either nitrogen-replete (800 uM ammonium) or medium lacking supplemental nitrogen. Samples were taken over a time series in order to characterize changes in physiology and gene expression during increasing nitrogen starvation. The two ecotypes' molecular responses to different nitrogen sources were also assessed by comparing gene expression of log phase cultures growing in ammonium vs. urea and cyanate (MED4), and vs. urea and nitrite (MIT9313).
Project description:Prochlorococcus marinus, the dominant photosynthetic organism in the ocean, is found in two main ecological forms: high-light-adapted genotypes in the upper part of the water column and low-light-adapted genotypes at the bottom of the illuminated layer. P. marinus SS120, the complete genome sequence reported here, is an extremely low-light-adapted form. The genome of P. marinus SS120 is composed of a single circular chromosome of 1,751,080 bp with an average G+C content of 36.4%. It contains 1,884 predicted protein-coding genes with an average size of 825 bp, a single rRNA operon, and 40 tRNA genes. Together with the 1.66-Mbp genome of P. marinus MED4, the genome of P. marinus SS120 is one of the two smallest genomes of a photosynthetic organism known to date. It lacks many genes that are involved in photosynthesis, DNA repair, solute uptake, intermediary metabolism, motility, phototaxis, and other functions that are conserved among other cyanobacteria. Systems of signal transduction and environmental stress response show a particularly drastic reduction in the number of components, even taking into account the small size of the SS120 genome. In contrast, housekeeping genes, which encode enzymes of amino acid, nucleotide, cofactor, and cell wall biosynthesis, are all present. Because of its remarkable compactness, the genome of P. marinus SS120 might approximate the minimal gene complement of a photosynthetic organism.
Project description:Metal-dependent superoxide dismutases (SODs) with a specific requirement for a manganese or iron ion for catalytic activity and copper- and zinc-dependent enzymes are essential for detoxification of superoxide anion radicals. Genome sequence analyses predict the existence of a nickel-dependent enzyme (NiSOD) as the unique SOD in oxygen-evolving marine cyanobacteria. NiSOD activity was observed in Escherichia coli when sodN and sodX (encoding a putative peptidase) from Prochlorococcus marinus MIT9313 were coexpressed.
Project description:Marine cyanobacteria of the genus Prochlorococcus represent numerically dominant photoautotrophs residing throughout the euphotic zones in the open oceans and are major contributors to the global carbon cycle. Prochlorococcus has remained a genetically intractable bacterium due to slow growth rates and low transformation efficiencies using standard techniques. Our recent successes in cloning and genetically engineering the AT-rich, 1.1 Mb Mycoplasma mycoides genome in yeast encouraged us to explore similar methods with Prochlorococcus. Prochlorococcus MED4 has an AT-rich genome, with a GC content of 30.8%, similar to that of Saccharomyces cerevisiae (38%), and contains abundant yeast replication origin consensus sites (ACS) evenly distributed around its 1.66 Mb genome. Unlike Mycoplasma cells, which use the UGA codon for tryptophane, Prochlorococcus uses the standard genetic code. Despite this, we observed no toxic effects of several partial and 15 whole Prochlorococcus MED4 genome clones in S. cerevisiae. Sequencing of a Prochlorococcus genome purified from yeast identified 14 single base pair missense mutations, one frameshift, one single base substitution to a stop codon and one dinucleotide transversion compared to the donor genomic DNA. We thus provide evidence of transformation, replication and maintenance of this 1.66 Mb intact bacterial genome in S. cerevisiae.
Project description:Prochlorococcus is a genus of marine cyanobacteria characterized by small cell and genome size, an evolutionary trend toward low GC content, the possession of chlorophyll b, and the absence of phycobilisomes. Whereas many shared derived characters define Prochlorococcus as a clade, many genome-based analyses recover them as paraphyletic, with some low-light adapted Prochlorococcus spp. grouping with marine Synechococcus. Here, we use 18 Prochlorococcus and marine Synechococcus genomes to analyze gene flow within and between these taxa. We introduce embedded quartet scatter plots as a tool to screen for genes whose phylogeny agrees or conflicts with the plurality phylogenetic signal, with accepted taxonomy and naming, with GC content, and with the ecological adaptation to high and low light intensities. We find that most gene families support high-light adapted Prochlorococcus spp. as a monophyletic clade and low-light adapted Prochlorococcus sp. as a paraphyletic group. But we also detect 16 gene families that were transferred between high-light adapted and low-light adapted Prochlorococcus sp. and 495 gene families, including 19 ribosomal proteins, that do not cluster designated Prochlorococcus and Synechococcus strains in the expected manner. To explain the observed data, we propose that frequent gene transfer between marine Synechococcus spp. and low-light adapted Prochlorococcus spp. has created a "highway of gene sharing" (Beiko RG, Harlow TJ, Ragan MA. 2005. Highways of gene sharing in prokaryotes. Proc Natl Acad Sci USA. 102:14332-14337) that tends to erode genus boundaries without erasing the Prochlorococcus-specific ecological adaptations.