The spc operon of Escherichia coli encodes 11
ribosomal proteins and SecY. The secY gene and downstream rpmJ encoding a
ribosomal protein, L36, are located distal to the promoter of the spc operon. It has been suggested that the stability of SecY
mRNA depends on rpmJ unless a rho-independent terminator is inserted immediately downstream of secY. Moreover, it has been suggested that RpmJ is dispensable for E. coli. We constructed rpmJ null strains, AY101 (DeltarpmJ::tetA) and AY201 (DeltarpmJ::cat), by replacing rpmJ with tetA, which encodes a
membrane protein responsible for tetracycline-resistance, and cat, which encodes a cytoplasmic
chloramphenicol acetyltransferase, respectively. Depletion of RpmJ did not inhibit
protein synthesis, whereas the growth of AY101 was defective at high temperatures. The level of SecY
mRNA decreased significantly in both disruptants even though the rho-independent terminator was inserted immediately downstream of secY. Some
periplasmic proteins were missing in the disruptants with a concomitant increase in the amount of phage
shock protein in the inner membrane. These phenotypes caused by the rpmJ null mutation were corrected by a plasmid carrying secY, but not by one carrying rpmJ.