Translation arrest of SecM is essential for the basal and regulated expression of SecA.
The SecM protein of Escherichia coli contains an arrest sequence (F(150)XXXXWIXXXXGIRAGP(166)), which interacts with the ribosomal exit tunnel to halt translation elongation beyond Pro-166. This inhibition is reversed by active export of the nascent SecM chain. Here, we studied the physiological roles of SecM. Arrest-alleviating mutations in the arrest sequence reduced the expression of secA, a downstream gene on the same mRNA. Among such mutations, the arrest-abolishing P166A substitution mutation on the chromosomal secM gene proved lethal unless the mutant cells are complemented with excess SecA. Whereas secretion defect due either to azide addition, a secY mutation, or low temperature leads to up-regulated SecA biosynthesis, this regulation was lost by a secM mutation, which synergistically retarded growth of cells with lowered secretion activity. Finally, an arrest-alleviating rRNA mutation affecting the constricted part of the exit tunnel lowered the basal level of SecA as well as its secretion defect-induced up-regulation. Thus, the arrest sequence of SecM has at least two roles in SecA translation. First, the transient elongation arrest in normal cells is required for the synthesis of SecA at levels sufficient to support cell growth. Second, the prolonged SecM elongation arrest under conditions of unfavorable protein secretion is required for the enhanced expression of SecA to cope with such conditions.
Institute for Virus Research and Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Kyoto University, Kyoto 606-8507, Japan.
Akiko
A
Murakami
Hitoshi
H
Nakatogawa
Koreaki
K
Ito
D000251:Adenosine Triphosphatases / Q000235:genetics* / Q000378:metabolism*; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism*; D001483:Base Sequence; D003080:Cold Temperature; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics* / Q000378:metabolism*; D015870:Gene Expression; D005798:Genes, Bacterial; D026901:Membrane Transport Proteins / Q000235:genetics* / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D014176:Protein Biosynthesis; D012270:Ribosomes / Q000378:metabolism; D000069816:SEC Translocation Channels; D000081416:SecA Proteins
D001426:Bacterial Proteins; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins