plant surface microbiology.pdf

136
Bernard R. Glick and Donna M. Penrose
K m values for the binding of ACC by ACC deaminase have been estimated
for enzyme extracts from 12 microorganisms at pH 8.5. These values ranged
from 1.5 to 17.4 mM (Honma and Shimomura 1978; Klee and Kishore 1992;
Honma 1993) indicating that the enzyme does not have a particularly high
affinity for ACC (Glick et al. 1999).
ACC deaminase activity has been induced in both Pseudomonas sp. strain
ACP and Pseudomonas putida GR12–2 by ACC, at levels as low as 100 nM,
(Honma and Shimomura 1978; Jacobson et al. 1994); both bacterial strains
were grown on a rich medium and then transferred to a minimal medium
containing ACC as its sole nitrogen source. The rate of induction, similar for
the enzyme from the two bacterial sources, was relatively slow: complete
induction required 8–10 h. Enzyme activity increased only approximately
tenfold over the basal level of activity even when the concentration of ACC
increased up to 10,000-fold.
Pyridoxal phosphate is a tightly bound cofactor of ACC deaminase in the
amount of approximately three moles of enzyme-bound pyridoxal phosphate
per mole of enzyme, or one mole per subunit (Honma 1985).
Genes encoding ACC deaminase have been cloned from a number of different
soil bacteria including Pseudomonas sp. strains 6G5 and 3F2 (Klee et al.
1991; Klee and Kishore 1992), Pseudomonas sp. strain 17 (Campbell and
Thomson 1996) Pseudomonas sp. strain ACP (Sheehy et al. 1991) and Enterobacter
cloacae strains CAL2 and UW4 (Glick et al. 1995; Shah et al. 1998);
yeast, Hansenula saturnus (Minami et al. 1998); and fungus, Penicillium citrinum
(Jia et al. 1999).
The ACC deaminase genes from Pseudomonas sp. strains 6G5 and F17, and
Enterobacter cloacae strains UW4 and CAL2 all have an ORF of 1014
nucleotides that encodes a protein containing 338 amino acids with a calculated
molecular weight of approximately 36.8 kDa (Klee et al. 1991; Campbell
and Thomson 1996; Shah et al. 1998). The genes from these strains are highly
homologous to each other: at the nucleotide level 6G5, F17, UW4 and CAL2
are 85–95 % identical to each other (Campbell and Thomson 1996; Shah et al.
1998) and most of the dissimilarities are in the wobble position (Shah et al.
1998). However, the DNA sequences from strains UW4 and CAL2 show only
about 74 % homology with the sequence of the ACC deaminase gene from
Pseudomonas sp. strain ACP (Sheehy et al. 1991; Shah et al. 1998).
Sequence data indicate that strain UW4 contains a DNA region similar to
that of the anaerobic transcription regulator, FNR, (fumarate and nitrate regulator)
at positions –39 to –49 (Grichko and Glick 2000). Moreover, the ACC
deaminase gene promoter in strain UW4 is under the transcriptional control
of a nearby gene that has a DNA sequence similar to a leucine-responsive regulatory
protein (LRP) and the LRP-like protein is transcriptionally regulated
by ACC (Grichko and Glick 2000; Li and Glick 2001).
When a broad host range plasmid containing the ACC deaminase gene
from Enterobacter cloacae UW4 was introduced into two nonplant growth-