The diversity of gas vesicle genes in Planktothrix rubescens from Lake Zürich

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Abstract
Part of the gas vesicle gene cluster was amplified by PCR from three strains of Planktothrix rubescens isolated from Lake Zürich, Switzerland. Each contains multiple alternating copies of gvpA and gvpC. All of the gvpA sequences in the different strains are identical. There are two types of gvpC: gvpC 20, of length 516 bp, encodes a 20 kDa protein of 172 amino acid residues (whose N-terminal amino acid sequence is homologous with the sequence of GvpC in Planktothrix [Oscillatoria] agardhii); gvpC 16, of length 417 bp, encodes a 16 kDa protein of 139 amino acid residues that differs in lacking an internal 33-residue section. An untranslated 72 bp fragment from the 3′ end of gvpC, designated ΩC, is also present in some strains. The two types of gvpC and presence of ΩC could be distinguished by the different lengths of PCR amplification products obtained using pairs of oligonucleotide primers homologous to internal sequences in gvpC and gvpA. Three genotype classes were found: GV1, containing only gvpC 20; GV2, containing gvpC 20 and ΩC; and GV3, containing gvpC 16, gvpC 20 and ΩC. Subclasses of GV2 and GV3 contained either one or two copies of ΩC. The accompanying paper by D. I. Bright & A. E. Walsby (Microbiology 145, 2769–2775) shows that strains of the GV3 genotype produce gas vesicles with a higher critical pressure than those of GV1 and GV2. A PCR survey of 185 clonal cultures of P. rubescens isolated from Lake Zürich revealed that 3 isolates were of genotype GV1, 73 were of GV2 and 109 were of GV3. The PCR technique was used to distinguish the gas vesicle genotype, and thence the associated critical-pressure phenotype, of single filaments selected from lakewater samples. Sequence analysis of the 16S rDNA and of regions within the operons encoding phycoerythrin, phycocyanin and Rubisco confirmed that these strains of Planktothrix form a tight phylogenetic group.