Differential colony formation of conditional lethal mutant strains on arabinose or glucose-containing LB agar plates.

The native promoters of 23 genes or operon pairs were replaced with the arabinose-inducible araBAD promoter (see Methods). By shifting these mutant strains from medium with 0.1% L-arabinose to medium without L-arabinose and supplemented with 0.4% D-glucose, expression of the essential gene was repressed. In all cases this resulted in growth inhibition or severe growth defects. The gene whose native promoter was replaced is indicated on the right hand side of each row; the ancestral strain TB741 is shown at the top. In the case of degS, substantial growth occurs even after repression, suggesting that growth only becomes inhibited once the gene products are sufficiently depleted; this has been observed previously [19]. This is also true, although to a lesser extent, for spoT, ftsK, and ygjD. To grow the spot plates, cultures were grown overnight in 0.1% arabinose, diluted into LB medium, and 5 µl of the indicated dilution were spotted onto plates and incubated at 37° for 10 hours. Those strains indicated with a cross were incubated for 24 hours.