CHARACTERIZATION AND EXPRESSION OF A GENE ENCODING SERINE TRNA(5) 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 en coding initiation factor IF1. Here we characterize this tRNA gene, nam ed serW. The serW gene expresses a minor form of serine tRNA(GGA), whi ch recognizes the most frequently used serine codons, UCC and UCU. Two promoters were identified by S1 nuclease mapping: P1, which lies abou t 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 cha racteristic of promoters regulated by ppGpp. A putative hairpin struct ure followed by a stretch of U residues about 25 nucleotides following the mature tRNA sequence resembles a rho-independent termination sign al. The upstream gene, infA, is followed by a transcriptional terminat or, but S1 mapping shows considerable readthrough. Thus serW expressio n appears to rely both on its own promoters and on promoters further u pstream. The downstream gene, encoding an unidentified protein of abou t 100 kDa, is expressed in the opposite orientation and also is follow ed by a termination signal. Therefore serW is expressed both as a mono cistronic gene and in combination with infA.