Kr. Gutshall et al., ANALYSIS OF A NOVEL GENE AND BETA-GALACTOSIDASE ISOZYME FROM A PSYCHROTROPHIC ARTHROBACTER ISOLATE, Journal of bacteriology, 177(8), 1995, pp. 1981-1988
We have characterized a new psychrotrophic Arthrobacter isolate which
produces beta-galactosidase isozymes. When DNA from this isolate was t
ransformed into an Escherichia coli host, we obtained three different
fragments, designated 12, 14, and 15, each encoding a different beta-g
alactosidase isozyme. The beta-galactosidase produced from fragment 12
was of special interest because the protein subunit was smaller (abou
t 71 versus 116 kDa) than those typically encoded by the lacZ family.
The isozyme encoded by fragment 12 was purified, and its activity and
thermostability were examined. Although the enzyme is highly specific
towards beta-D-galactoside substrates, its levels in the isolate do no
t increase in cells grown with lactose. Nucleotide sequence determinat
ion showed that the gene encoding isozyme 12 is not similar to the oth
er members of the lacZ family but has regions similar to beta-galactos
idase isozymes from Bacillus stearothermophilus and B. circulans. Addi
tion of the isozyme 12 sequence to the database made it possible to ex
amine these enzymes as possible members of a new, separate family. Our
analysis of this new family showed some conserved amino acids corresp
onding to the lacZ acid-base catalytic region but no homology with the
nucleophilic region. On the basis of these comparisons, we designated
this a new lacG family.