Genes coding for SecG and Leu2-tRNA form an operon to give an unusual RNA comprising mRNA and a tRNA precursor.
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 degrees 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.
Nishiyama
Ken-ichi
K
Institute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. unishiy@mail.ecc.u-tokyo.ac.jp
Tokuda
Hajime
H
D000595:Amino Acid Sequence; D001483:Base Sequence; D015152:Blotting, Northern; D004926:Escherichia coli / Q000235:genetics; D029968:Escherichia coli Proteins / Q000235:genetics*; D015964:Gene Expression Regulation, Bacterial; D008565:Membrane Proteins / Q000235:genetics*; D008969:Molecular Sequence Data; D009876:Operon; D011401:Promoter Regions, Genetic / Q000235:genetics*; D012322:RNA Precursors / Q000235:genetics* / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D012329:RNA, Bacterial / Q000235:genetics; D012356:RNA, Transfer, Leu / Q000235:genetics*; D000069816:SEC Translocation Channels; D014158:Transcription, Genetic
D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D012322:RNA Precursors; D012329:RNA, Bacterial; D012356:RNA, Transfer, Leu; D000069816:SEC Translocation Channels; C088616:SecG protein, E coli