The Bacillus subtilis anti-TRAP
protein regulates the ability of the
tryptophan-activated TRAP
protein to bind to trp operon leader
RNA and promote transcription termination. AT synthesis is regulated both transcriptionally and translationally by uncharged
tRNA(Trp). In this study, we examined the roles of AT synthesis and
tRNA(Trp) charging in mediating physiological responses to
tryptophan starvation. Adding excess
phenylalanine to wild-type cultures reduced the charged
tRNA(Trp) level from 80% to 40%; the charged level decreased further, to 25%, in an AT-deficient mutant. Adding
tryptophan with
phenylalanine increased the charged
tRNA(Trp) level, implying that
phenylalanine, when added alone, reduces the availability of
tryptophan for
tRNA(Trp) charging. Changes in the charged
tRNA(Trp) level observed during growth with added
phenylalanine were associated with increased transcription of the genes of
tryptophan metabolism. Nutritional shift experiments, from a medium containing
tryptophan to a medium with
phenylalanine and
tyrosine, showed that wild-type cultures gradually reduced their charged
tRNA(Trp) level. When this shift was performed with an AT-deficient mutant, the charged
tRNA(Trp) level decreased even further. Growth rates for wild-type and mutant strains deficient in AT or TRAP or that overproduce AT were compared in various media. A lack of TRAP or overproduction of AT resulted in
phenylalanine being required for growth. These findings reveal the importance of AT in maintaining a balance between the synthesis of
tryptophan versus the synthesis of
phenylalanine, with the level of charged
tRNA(Trp) acting as the crucial signal regulating AT production.