The gene coding for flavodoxin from Anabaena PCC 7119 was cloned by using the polymerase chain reaction (PCR). The gene is transcribed into a 1250-base transcript. The expression of the flavodoxin gene was analysed and found to be regulated at the transcriptional level by the availability of iron. The PCR-amplified gene was cloned into the expression vector pTrc 99b and expressed in Escherichia coli. High concentrations of flavodoxin were found (20% of total protein). The recombinant protein was purified from the cytosolic fraction of the cells and it exhibited properties identical with those of the wild-type Anabaena flavodoxin.

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Six strains of unicellular cyanobacteria were examined for the presence of plasmids. Analysis of lysates of these strains by CsCl-ethidium bromide density centrifugation yielded a major chromosomal DNA band and a minor band containing covalently closed circular plasmid DNA, as shown by electron microscopy and agarose gel electrophoresis. The sizes of the various plasmid species were determined; in each of the Synechococcus strains 6301, 6707, and 6908 two plasmid species were found with molecular weights of 5.3 x lo6 and 32.7 x 109. Synechococcus strain 7425 had two plasmids of molecular weight 5.4 x lo6 and 24 x 106. Synechococcus strain 6312 and Synechocystis strain 7005 each contained one plasmid species with molecular weight of 15.9 x 106 and 2.0 x lOg, respectively. Restriction enzyme analysis revealed identical cleavage patterns for the plasmids of identical molecular weight.

We have used the TEM beta-lactamase transposon Tn901, located on Escherichia coli plasmid pRI46, to introduce in vivo a genetic marker into plasmid pUH24, present in the cyanobacterial strain Anacystis nidulans R-2. Restriction enzyme analysis and heteroduplex studies of the 8.3 x 10(6)-dalton plasmids pCH1-pCH5, present in the ampicillin-resistant A. nidulans R-2 colonies obtained after transformation with pRI46, demonstrated that these plasmids consist of the complete sequence of Tn901 inserted at different places into plasmid pUH24. The pUH24::Tn901 recombinant plasmids transform A. nidulans R-2 with a frequency of 10(-4)--10(-5) per microgram of plasmid DNA and contain a single cleavage site for the restriction enzyme Xho I. From pCH1 a plasmid of 5.5 x 10(6) daltons,pUC1, was constructed with only a part of the Tn901 sequence and an additional single cleavage site for the restriction enzyme BamHI. This plasmid, as well as plasmids pCH1-pCH5, are potentially useful as vectors for cloning genes in cyanobacteria and for studying cyanobacterial plasmid biology.

Two plasmids were constructed consisting of the E. coli vector pACYC184 and the cyanobacterial plasmid pUC1. These recombinants, designated pUC104 and pUC105, can be transformed to E. coli K12 as well as to the cyanobacterium Anacystis nidulans R2 and in both hosts they express their antibiotic markers. pUC104 and pUC105 differ with respect to the location and the orientation of the pACYC184 segment in pUC1. pUC104 was found to be stable under all circumstances. Transformation of pUC105 to A. nidulans R2 gave intact plasmids when chloramphenicol was the selective agent, but upon ampicillin selection a deletion derivative was produced identical to pUC1. Further characteristics of pUC104 and pUC105 are described and their usefulness as cloning vectors is discussed.