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J. Bacteriol., May 1997, 2884-2891, Vol 179, No. 9
X Xu, I Khudyakov and CP Wolk
Fox- mutants of Anabaena sp. strain PCC 7120 are unable to fix dinitrogen
in the presence of oxygen. A fragment of the DNA of Anabaena sp. was cloned
by complementation of a spontaneous Fox-, cyanophage- resistant mutant,
R56, and characterized. Random insertion of transposon Tn5 delimited the
complementing DNA to a 0.6-kb portion of the cloned fragment. Sequencing of
this region and flanking DNA showed one complete open reading frame (ORF)
similar to the gene rfbP (undecaprenyl-phosphate
galactosephosphotransferase) and two partial ORFs similar to genes rfbD
(GDP-D-mannose dehydratase) and rfbZ (first mannosyl transferase), all of
which are active in the synthesis of the O antigen unit of the
lipopolysaccharide (LPS) component of the outer membrane of gram-negative
bacteria. In a transposon (Tn5-1087b)- induced, Fox-, cyanophage-resistant
mutant, B14, the transposon was found within the same rfbP-like ORF. The
three ORFs were insertionally inactivated with the omega cassette (P.
Prentki and H. M. Krisch, Gene 29:303-313, 1984) or with Tn5::omega. Only
the insertions in the rfbZ- and rfbP-like ORFs led to resistance to
cyanophages A-1(L) and A-4(L) and to a Fox- phenotype. Electrophoretic
analysis showed that interruption of the rfbZ- and rfbP-like ORFs resulted
in a change in or loss of the characteristic pattern of the lengths of the
LPS, whereas interruption of the rfbD-like ORF merely changed the
distribution of the lengths of the LPS to one with a greater prevalence of
low molecular weights. According to electron microscopy, interruption of
the rfbP-like ORF may have led to aberrant deposition of the layers of the
heterocyst envelope, resulting in increased leakage of oxygen into the
heterocyst. The results suggest that modified LPS may prevent cyanophage
infection of Anabaena sp. vegetative cells and the formation of a
functional heterocyst envelope.
Copyright © 1997, American Society for Microbiology
Lipopolysaccharide dependence of cyanophage sensitivity and aerobic nitrogen fixation in Anabaena sp. strain PCC 7120
MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing 48824, USA.
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