Genes coding for SecG and Leu2-tRNA form an operon to give an unusual RNA comprising mRNA and a tRNA precursor

Abstract

The secG gene encoding the SecG subunit of the SecYEG translocon and the leuU gene encoding Leu2-tRNA are very closely located on the Escherichia coli chromosome. A secG–leuU disruptant was not viable unless secG–leuU was induced from a plasmid, indicating that leuU is an essential gene since secG is dispensable at 37 °C. A mutant strain in which the promoter region for secG was replaced with cat revealed the same phenotype as the secG–leuU disruptant, indicating that leuU was expressed from the secG promoter. When the secG–leuU locus was placed on a high copy plasmid, an RNA comprising both mRNA for SecG and a precursor for Leu2-tRNA was detected on a Northern blot. Moreover, a secG–leuU transcript was amplified by RT-PCR using the total RNA fraction prepared from wild type E. coli cells but not from the secG–leuU and the secG promoter disruptants, indicating that secG–leuU forms an operon. Thus, the expression of Leu2-tRNA requires expression of the upstream secG gene. The gene structure of secG–leuU was conserved among Gram-negative bacteria, although the sequences separating the two genes were quite diverse. The physiological significance of this unusual gene organization is discussed.