Mutational and in vivo methylation analysis of F-factor PifC protein binding to the pif operator and the region containing the primary origin of mini-F replication

Analysis in vitro and in vivo of the transcriptional regulator CrgA of Neisseria meningitidis upon contact with target cells

Contact between CrgA, a LysR-like regulatory protein in Neisseria meningitidis, and DNA is involved in the repression of several bacterial genes upon contact with epithelial cells. We used a defined in vitro system containing crgA promoter, purified RNA polymerase (RNAP) and purified CrgA protein to demonstrate that CrgA was directly responsible for this transcriptional repression. Interaction between the C-terminal domain of CrgA and the RNAP led to the production of short abortive transcripts, suggesting that CrgA may act by preventing RNAP from clearing the promoter. We probed the regulation by CrgA of its own production by analysing CrgA-DNA contacts during cell-bacteria interaction by assaying in vivo protection against dimethyl sulphate (DMS) methylation. Comparison of DMS footprints in vitro and in vivo suggested that CrgA repressed transcription through specific base contacts, probably in the major groove of the DNA double helix, resulting in DNA looping. Upon contact with target cells, CrgA was released from the DNA, allowing transcription of the target gene to proceed to elongation and facilitating tight control of the expression of genes regulated by CrgA.

Role of tumor necrosis factor receptors TNFR-I (P55) and TNFR-II (P75) in corneal transplantation

BACKGROUND

To determine the role of tumor necrosis factor-alpha (TNF-alpha) receptor (TNFR) function in corneal allograft immunology.

METHODS

Animals with gene-targeted deficiency in TNFR-I (p55-/-), TNFR-II (p75-/-), or combined TNFR-I/TNFR-II deficiency (p55-/-p75-/-) and their wild-type controls were used as recipients of fully-mismatched (BALB/c; n=88) or multiple minor alloantigen-mismatched (BALB.b; n=62) orthotopic corneal transplants to determine the effect of selective deficiency in one or both TNF-alpha receptors on corneal allograft survival. Grafted recipients were followed biomicroscopically for signs of rejection, and survival data were analyzed by the Kaplan-Meier method.

RESULTS

There was no discernible difference in survival of fully-mismatched BALB/c corneal grafts in p55-/- (n=12; P=0.76) or in double-knockout p55-/-p75-/- (n=13; P=0.41) as compared with wild-type C57BL/6.129 hosts. However, the survival of BALB/c allografts was lower in p75-/- (n=10; median survival 20 days) as compared with control C57BL/6 (n=30; median survival 30 days) hosts (P=0.02). In contrast, there was no discernible effect in survival of minor alloantigen-disparate BALB.b corneal grafts in p75-/- (n=13; P=0.95) or in combined p55-/-p75-/-(n=10; P=0.17) hosts as compared with C57BL/6 (n=9) and C57BL/6.129 (n=10) wild-type controls, respectively. However, there was a profound enhancement in the survival of BALB.b allografts in p55-/- recipients (n= 10; median survival 35 days) as compared to wild-type C57BL/6.129 (n=10; median survival 25 days) controls (P<0.01).

CONCLUSIONS

Our data suggest that the two TNF-alpha receptors largely play discrete roles in mediating rejection of murine corneal allografts. TNFR-I (p55) function seems to be integral to the rejection of minor-disparate grafts, and its selective suppression leads to enhancement of allograft survival. In contrast, TNFR-II (p75) function appears to be associated with enhanced survival of major histocompatibility complex-disparate allografts. The combined deletion of TNFR functionality in p55-/-p75-/- confers no net advantage or disadvantage to major histocompatibility complex or minor alloantigen-disparate grafts.

Both the fipA gene of pKM101 and the pifC gene of F inhibit conjugal transfer of RP1 by an effect on traG

The mechanisms by which gene products inhibit the conjugal transfer of IncP plasmids (e.g., RP1) have been little studied. We have isolated and characterized one such gene, fipA (624 nucleotides), from the SmaI (14.8 kb)-AatII (15.6 kb) region of pKM101(IncN). This gene, which is also conserved in other IncN plasmids, is transcribed in an anticlockwise direction, probably as part of a transfer operon that includes traHI. The FipA protein (24 kDa) appears to be cytoplasmic and, when expressed from a multicopy plasmid, retards the growth of Escherichia coli WP2. The mode of action of fipA was compared with that of the apparently unrelated pifC gene from F(IncFI). Both genes inhibit the transfer of IncPalpha and IncPbeta plasmids but to different degrees. They also inhibit the mobilization of RSF1010 (which requires the RP1 pilus genes and traG) but not of CloDF13 (which encodes a traG homolog). Evidence that traG was the specific target of inhibition was obtained in an artificial system in which cloned traG was used to enhance RSF1010 mobilization via the N pilus system. Such enhancement did not occur in the presence of fipA or pifC. The availability of an in vivo assay of PifC enabled us to show that F pif operon expression increased in cells carrying F'lac and traG, but only if the traG coding sequence was intact. This finding suggested that conjugal inhibition of RP1 was most likely due to a PifC-TraG protein interaction. On phenotypic grounds inhibition of traG by fipA is also likely to occur posttranscriptionally. Whether or not the selection of traG as the inhibition target is an evolutionary tactic to limit the spread of P plasmids, we anticipate that fipA and pifC will prove useful in further investigation of the conjugal roles of traG and its homologs.

The miniF plasmid C protein: sequence, purification and DNA binding

The C (pifC) protein of miniF represses transcription of its own gene by binding to the pif operator (pifO); it is also needed for replication initiated from the miniF primary origin (ori-1). We have determined the nucleotide sequence of the C gene. The gene has been inserted into an expression vector under Ptrp control where it is expressed at high levels. The C protein has been purified from cells carrying the Ptrp-C plasmid, and a preliminary study of C protein-DNA binding properties has been carried out. C protein binds strongly to pifO, and weakly to sequences in the ori-1 region.