Beginning a transcriptional analysis of the leading region in F plasmid DNA transfer

Nucleotide sequence and transcriptional analysis of a third function (Flm) involved in F-plasmid maintenance

The leading region of the conjugative F plasmid encodes for a function, Flm, capable of extending the maintenance of normally unstable plasmids. Nucleotide sequencing and functional studies of flm locus have shown that it consists of at least two genes, flmA and flmB, which are physically and functionally homologous to hok and sok of parB in plasmid R1. The 52-amino acid flmA-coded polypeptide is almost identical to the hok product which has been shown to be a membrane-associated lethal protein [Gerdes et al., EMBO J. 5 (1986) 2023-2029]. Gene flmB codes for a 100 nucleotide, non-translated, complementary RNA which overlaps the 5' leader sequence of the flmA RNA. The flmA RNA also encodes an open reading frame (ORF70) which overlaps the flmA-coding sequence and may be a third gene involved in the Flm function. S1 analysis and functional studies suggest that the antisense flmB RNA binds to the flmA RNA and suppresses the expression of the lethal product, presumably by blocking coupled translation of ORF70 and flmA. Secondary structure analysis predicts that the flmA RNA is extremely stable compared to the regulatory flmB RNA. We suggest that when these RNA species are retained by cells which have lost the F plasmid, the more stable flmA RNA will eventually be translated thus leading to cell death. This phenomenon provides a third mechanism, additional to ParFIA and Ccd functions, to ensure maintenance of the F plasmid in a growing bacterial population.

Unrelated conjugative plasmids have sequences which are homologous to the leading region of the F factor

Conjugative plasmids from various incompatibility groups which carry DNA homologous to the ssb gene of the F factor were found to have additional homology with the F factor. This region homologous with F was located on both sides of the ssb gene and occupied a considerable part of the leading region, i.e., the 12.9-kilobase portion of F transferred first during conjugation. This region was the only region of the F factor which has a homologous counterpart on many plasmids.

Cloning and analysis of pif, replication and leading regions of the F plasmid

We describe the molecular cloning of BglII fragments of the hybrid plasmid pRS5 (pSC101 and EcoRI fragments of F; f7, f5, f3 and f6). The clones isolated were examined for the expression of F-specified replication, incompatibility, mobilization and inhibition of T7 bacteriophage multiplication. Proteins directed by the BglII clones were labelled in Escherichia coli K12 maxicells and analyzed by SDS-polyacrylamide gel electrophoresis. The sizes of previously reported proteins, encoded by the replication, incompatibility and leading regions encompassed by these plasmids have been confirmed in this study. In addition, the results demonstrate that a pif gene, which encodes an 80,000 dalton polypeptide essential for the inhibition T7 phage multiplication, is located on the BglII fragment that spans the junction of EcoRI fragments f7 and f5.

Stabilization of the cloning vector pACYC184 by insertion of F plasmid leading region sequences

The leading region of the F plasmid is, by definition, the first part of the plasmid DNA to be transferred to the recipient cell during conjugation. Restriction fragments of the leading region, when cloned into the plasmid vector pACYC184, extended the maintenance of the normally unstable p15A-derived vector replicon in rec+ Escherichia coli K-12 cells. Mutations in the host's general recombination systems were found to influence the maintenance of these hybrid plasmids.

Transcriptional analysis of the leading region in F plasmid DNA transfer

Transcriptional activity associated with the leading region (53.8-66.7F) in F DNA transfer has been shown by RNA-DNA hybridization studies to occur on the anterior segment extending from 59.4 to 66.7F. Promoter-probe analysis of cloned leading region segments detected two promoters within the transcribed portion of the leading region. The promoter active across the 64.7F EcoRI site on the transferred F strand was associated with the expression of two polypeptides, 6d and 13.5p, located between 64.7-66.6F. However, no definite role could be ascribed to the second promoter operative through the 66.6F Bg/II site located in close proximity to oriT, the origin of transfer.

Cloning and polypeptide analysis of the leading region in F plasmid DNA transfer

A segment of the F plasmid DNA, located between the origin of transfer and the primary F replication region, is the first to enter the recipient cell during conjugation. PstI, SalI, and SmaI restriction endonuclease sites have been mapped within this leading region in conjugational DNA transfer and chimeric plasmids carrying overlapping fragments of the region have been constructed. Analyses of polypeptides of Mr 27,800, 23,100, 14,400, and 11,000 to be encoded by sequences within the leading region.