The genes encoding bacterial luciferase and the other proteins required for bioluminescence are situated in lux operons, which have been most studied in Photobacterium and Vibrio. In Vibrio fischeri, all lux genes can be assigned to two operons, L and R, whose organization is illustrated in Figure 5.2.4 The only known gene in operon L is luxR, which encodes a regulatory protein. Operon R contains luxA and luxB, which code for the a and |3 subunits of luciferase respectively, and luxC, D, E and I, which respectively code for a fatty acid reductase, an acyl transferase, an acylprotein synthase and synthesis cf the 'autoinducer', N-(3-oxo-hexanoyl) homoserine lactone, which induces light production in all V. fischeri strains. In V. fischeri strains MJ-1 and ATCC7744 these genes are in the orier luxICDABE. The organization of structural genes for bioluminescence in other luminescent bacteria is similar, except that regulatory genes are omitted.5 Another gene, luxF, lying between luxB and luxE and sharing high homdogy with luxB, has been identified in Photobacterium phosphoreum and P leiognathi.6,7 An additional gene, luxG, has been identified downstream of luxE in all marine bioluminescent bacteria, but not in any Photorhabdus luminescens
Fig. 5.1. The mechanism of bacterial bioluminescence. L represents luciferase, roman numerals show key intermediates, * indicates excited intermediate.
Fig. 5.2. A physical map of the lux regulon.
strains studied,5 and gene luxH has been identified downstream of luxG in V harveyi.8 These genes are not essential for a bioluminescent phenotype but may be involved in the generation of reduced flavin substrate for the luminescence reaction.
All luxA and luxB genes sequenced (e.g., refs. 7,9) show a high degree of sequence homology. The luxA gene shows greater conservation than luxB, consistent with the postulate that the a subunit controls the kinetic properties of the enzyme. Sequence analysis indicates that bacterial luciferases may be subdivided into two distinct classes, the first represented by those from V. fischeri and Photobacterium and the second by V. harveyi, P. luminescens and a symbiont from the fishKryptophanaron alfredi, the latter group comprising the less heat-lable enzymes. The high homology between a and |3 subunits both within and between different species indicates that luxA and luxB arose by gene duplica-tion.5 The a subunit appears to be solely responsible for the catalytic activity of the enzyme but the | subunit is essential for functionality, and may be required to maintain the catalyti-cally active conformation of a residues.
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