MYXOCOCCUS-XANTHUS SASN ENCODES A REGULATOR THAT PREVENTS DEVELOPMENTAL GENE-EXPRESSION DURING GROWTH

Myxococcus xanthus multicellular fruiting body development is initiate d by nutrient limitation at high cell density. Five clustered point mu tations (sasB5, -14, -15, -16, and -17) can bypass the starvation and high-cell-density requirements for expression of the 4521 developmenta l reporter gene. These mutants express 4521 at high levels during grow th and development in an asgB background, which is defective in genera tion of the cell density signal, A signal. A 1.3-kb region of the sasB locus cloned from the wild-type chromosome restored the SasB(+) pheno type to the five mutants. DNA sequence analysis of the 1.3-kb region p redicted an open reading frame, designated SasN. The N terminus of Sas N appears to contain a strongly hydrophobic region and a leucine zippe r motif. SasN showed no significant sequence similarities to known pro teins. A strain containing a newly constructed sasN-null mutation and Ohm 4521 Tn5lac in an otherwise wild-type background expressed 4521 at a high level during growth and development. A similar sasN-null mutan t formed abnormal fruiting bodies and sporulated at about 10% the leve l of wild type. These data indicate that the wild-type sasN gene produ ct is necessary for normal M. xanthus fruiting body development and fu nctions as a critical regulator that prevents 4521 expression during g rowth.