SUMMARYFluorescent proteins (FPs) have become essential tools for a growing number of fields in biology. However, such tools have not been widely adopted for use in microalgal research. The aim of this study was to express and compare six FPs (blue mTagBFP, cyan mCerulean, green CrGFP, yellow Venus, orange tdTomato and red mCherry) in the popular model microalga Chlamydomonas reinhardtii. To circumvent the transgene silencing that often occurs in C. reinhardtii, the FPs were expressed from the nuclear genome as transcriptional fusions with the sh-ble antibiotic resistance gene, with the foot and mouth disease virus 2A self-cleaving sequence placed between the coding sequences. All ble-2A-FPs tested are well-expressed and efficiently processed to yield mature, unfused FPs that localize throughout the cytoplasm. The fluorescence signals of each FP were detectable in whole cells by fluorescence microplate reader analysis, live-cell fluorescence microscopy, and flow cytometry. Furthermore, we report a comparative analysis of fluorescence levels relative to auto-fluorescence for the chosen FPs. Finally, we demonstrate that the ble-2A expression vector may be used to fluorescently label an endogenous protein (a-tubulin). We show that the mCerulean-a-tubulin fusion protein localizes to the cytoskeleton and flagella, as expected, and that cells containing this fusion protein had normal cellular function. Overall, our results indicate that, by use of the ble-2A nuclear expression construct, a wide array of FP tools and technologies may be applied to microalgal research, opening up many possibilities for microalgal biology and biotechnology.

The X-type primosome was discovered during the resolution and reconstitution in vitro of the complementary strand DNA replication step of the X174 viral life cycle. This multienzyme bidirectional helicase-primase complex can provide the DNA unwinding and Okazaki fragment-priming functions at the replication fork and has been implicated in cellular DNA replication, repair, and recombination. We have used gel mobility shift assays and enhanced chemiluminescence Western analysis to isolate and identify the pathway of primosome assembly at a primosome assembly site (

Gel filtration chromatography was used to isolate both preprimosomal and primosomal complexes formed on single-stranded DNA-binding protein-coated X174 DNA by the combination of PriA, PriB, PriC, DnaT, DnaB, DnaC, and DnaG. The presence and relative amounts of primosomal proteins in these complexes were determined by Western blotting. Protein-DNA complexes isolated (i) after assembly in the presence of 10 M ATP, (ii) after preprimosome movement in the presence of 1 mM ATP, (iii) after priming in the presence of the four ribonucleoside triphosphates, or (iv) after complementary strand DNA replication in the presence of the DNA polymerase III holoenzyme all had the same protein composition; preprimosomes contained PriA, PriB, PriC, DnaT, and DnaB, whereas primosomes included DnaG. The stable association of DnaG with the protein-DNA complex could be attributed partially to its ability to remain bound to the primers synthesized. In the absence of PriC, the efficiencies of priming and replication were reduced by one-third and one-half, respectively, even though PriC was not required for the formation of stable protein-DNA complexes on a 304-nucleotide-long single strand of DNA containing a primosome assembly site (Ng, J. Y., and Marians, K. J. (1996) J. Biol. Chem. 271, 15642-15648). We hypothesize that maintenance of the primosome on the replicated DNA may provide a mechanism to allow primosomes to participate in the resolution of recombination intermediates and intermediates formed during double strand break repair by permitting the re-establishment of a replication fork.Although the X174-type primosome was discovered over 20 years ago (1, 2) because of its involvement in bacteriophage DNA replication, its functions in normal cellular processes have only recently begun to surface (3-8). In order to expand our understanding of the cellular roles played by this multienzyme DNA unwinding and priming replication machine, we have examined the mechanisms of primosome assembly and the subsequent enzymatic reactions catalyzed by it in detail.There are three main stages in the replication cycle of X174 DNA (9). The first stage is the conversion of the viral singlestranded circular (ss(c)) 1 DNA to the duplex replicative form (ss(c) 3 RF) initiated by the X primosome (1, 2). Seven primosomal proteins, PriA, PriB, PriC, DnaB, DnaC, DnaG, and DnaT, are required for the assembly of a primosome at the primosome assembly site (PAS) on a single-stranded DNAbinding protein (SSB)-coated X174 phage DNA (1, 2, 10 -13). This multienzyme complex can translocate in both the 3Ј 3 5Ј and 5Ј 3 3Ј directions along single-stranded DNA (10) and is able to utilize ribonucleoside triphosphates to synthesize multiple primers at random sites along the viral DNA (2, 10, 15, 16). Chain elongation by the DNA polymerase III holoenzyme (pol III HE) then converts the single-stranded viral genome to RFII DNA. The first (17) and third (18) articles in this series address the mechanism of, and the role of PriB in, the assembly of the X-type primosome, resp...

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