STRUCTURE AND ORGANIZATION OF THE GAS VESICLE GENE-CLUSTER ON THE HALOBACTERIUM-HALOBIUM PLASMID-PNRC100

Halobacterium halobium strain NRC-1 contains intracellular gas-filled vesicles (GVs) that confer buoyancy to the cells. Cloning of the major GV protein (GvpA)-encoding gene, gvpA, and analysis of GV-deficient mutants (Vac-) of H. halobium led to the identification of a region of a 200-kb plasmid, pNRC100, important for GV synthesis. We report here the nucleotide sequence of an 8520-bp region which, including gvpA, contains twelve open reading frames (ORFs) that are organized into two divergent transcription units, gvpAC oriented rightward, and gvpD, E, F, G, H, I, J, K, L, and M located upstream from gvpAC and oriented leftward. Insertions into the gvpA promoter and gvpD and E resulted in the Vac- phenotype. The overall gene organization is highly compact with the end of one ORF overlapping with the beginning of the next in most cases. The gene cluster is bracketed by two ISH8 element copies in inverted orientation, an organization suggestive of a composite transposon. Comparison of predicted amino acid sequences showed homology between GvpA, and the gvpJ and gvpM putative gene products. The putative gvpC gene product contains eight copies of an imperfectly repeated sequence with similarity to repeats in a cyanobacterial GvpC plus a highly acidic C-terminal region not found in the cyanobacterial homologue.