Characterization and expression of a gene encoding serine tRNA5 from Escherichia coli

The genes for translational components frequently are located together on the Escherichia coli genome. We have reported previously that the gene for a serine tRNA lies directly downstream from infA, the gene encoding initiation factor IF1. Here we characterize this tRNA gene, named serW. The serW gene expresses a minor form of serine tRNA(GGA) which recognizes the most frequently used serine codons, UCC and UCU. Two promoters were identified by S1 nuclease mapping: P1, which lies about 72 bp upstream from the structural gene; and P2, which lies about 35 bp upstream. Expression from P1 and P2 is comparable under conditions of rapid growth. The P2 promoter is followed by a GC-rich element characteristic of promoters regulated by ppGpp. A putative hairpin structure followed by a stretch of U residues about 25 nucleotides following the mature tRNA sequence resembles a rho-independent termination signal. The upstream gene, infA, is followed by a transcriptional terminator, but S1 mapping shows considerable readthrough. This serW expression appears to rely both on its own promoters and on promoters further upstream. The downstream gene, encoding an unidentified protein of about 100 kDa, is expressed in the opposite orientation and also is followed by a termination signal. Therefore serW is expressed both as a monocistronic gene and in combination with infA.