Spontaneous and reversible high-frequency frameshifts originating a phase transition in the carotenoid biosynthesis pathway of the phototrophic bacterium Rhodobacter sphaeroides 2.4.1.

The synthesis of carotenoids in strain 2.4.1 of the phototrophic bacterium Rhodobacter sphaeroides is spontaneously turned on and off at a high frequency (10(-5) per cell per generation) giving rise alternatively to red (wild type) and green (mutant) clones. The crtD gene is not functional in green mutants as a consequence of the spontaneous addition of a guanosine in a stretch of seven guanosines located in the 5'-terminal coding region of this gene originating a frameshift. All spontaneous wild-type revertants isolated from green mutants had recovered the crtD gene function by loss of one of these reiterated guanosines. The transition Crt(+)----Crt(-)----Crt+, is strain-dependent, since Crt+ clones were not detected in ethyl methane sulphonate (EMS)-induced CrtD- mutants of two other strains of R. sphaeroides (WS22 and RS630) which harbour a recombinant plasmid containing the crtD gene from a spontaneous CrtD- mutant of strain 2.4.1 of R. sphaeroides.