PIGMENTATION PHENOTYPE INSTABILITY IN MYXOCOCCUS XANTHUS

Abstract

When M. xanthus FB.is plated on CTE or PTE agar, 2-10% of the colonies are yellow (y phenotype); the majority is tan (t). Cells from most t colonies produce t and y colonies; cells from most y colonies produce only those of the y phenotype. Apparently color-stable strains have been isolated; however, these de-stabilize during serial sub-culture. The y and t phenotype are also distinguishable by differences in colony morphology and fruiting behavior. Fluctuation test experiments demonstrate that conversion of t cells to the y phenotype occurs at random in suspension culture. These and other experiments indicate that the conversion to y is enhanced in low cell density culture ( <10°/m) and is suppressed at higher cell density. An extra-cellular inhibitor of expression of the y phenotype is present in hiah density FB cultures. Ultraviolet irradiation of FB results in an increase in the y:t ratio with exposure time. This is due to conversion of cells from the t to the y phenotype, as shown by sectored colonies and equal UV-sensitivity of temporarily stable t and y strains. Low level photoreactivation occurs. Exposure to mitomycin C (5 g/ml) or to nalidixic acid (20 pg/ml) and growth at 36.5 C all increase the y:t ratio. Acridine orange has no effect. Chloramphenicol (25 ig/ml) does not inhibit the mitomycin-induced conversion to y. A highly polar, yellow pigment (abs. max. 379 nm in methanol) accounts for the color difference. Synthesis of this pigment is inhibited at 36.5 C, the maximum growth temperature of the organism.