SUMMARY: An Escherichia coli K12 mutant defective in both serine biosynthesis (serA) and glycine transport (cycA) was found to exhibit a glycine cleavage negative (GCV) phenotype, i.e. was unable to use glycine as a serine source. While [2-C]glycine uptake and induction of a λgcvT::lacZ fusion were greatly reduced in a cycA mutant compared to the wild-type, both strains exhibited parallel increases in uptake and induction with increasing exogenous glycine concentrations. A plasmid carrying the wild-type cyc region complemented the GCVphenotype and restored both glycine uptake and glycine-inducible gcvT::lacZ expression. Wild-type and cycA strains grown in the presence of either a glycine-containing tripeptide or threonine, which can be degraded internally into glycine, exhibited similar induction of the gcvT::lacZ fusion. However, when a gcv mutation, which causes glycine to accumulate within the cell, was introduced into the cycA strain, there was increased induction of the gcvT::lacZ fusion, but induction was less than that observed in a gcv cycA strain. It is proposed that cyc serves primarily in the regulation of gcv by transporting glycine into the cell, which endogenously induces gcv expression. However, the possibility of some form of exogenous regulation of gcv, mediated by the cyc-encoded glycine transport system, exists.


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