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J. Bacteriol., 08 1995, 4730-4741, Vol 177, No. 16
V Athanasopoulos, J Praszkier and AJ Pittard
A 2,385-bp sequence that contains the information for the autonomous
replication of the IncL/M plasmid pMU604 was characterized. Genetic
analyses revealed that the replicon specifies at least four structural
genes, designated repA, repB, repC, and rnaI. The repA gene encodes a
protein with a molecular weight of 40,861 which probably functions as an
initiator for replication. The functions of the proteins of the repB and
repC genes are unclear; however, mutations in the start codon of repB
reduced the expression of both repB and repA, indicating that these two
genes are translationally coupled. The rnal gene encodes a small antisense
RNA of about 75 to 77 bases and is responsible for the incompatibility
phenotype, thus implicating its role as the main copy number determinant.
RNAI exerts its effect in trans to repress the expression of repA at the
posttranscriptional level. Furthermore, two complementary sequences of 8
bases, with the potential to interact and form a putative pseudoknot
structure, were identified in the leader region of the repA mRNA.
Base-pairing between the two complementary sequences was shown to be
critical for efficient repA expression. A model for the regulation of
pMU604 replication involving both translational coupling and pseudoknot
formation is proposed.
Copyright © 1995, American Society for Microbiology
The replication of an IncL/M plasmid is subject to antisense control
Department of Microbiology, University of Melbourne, Parkville, Victoria, Australia.
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