The archaeal halophilic virus-encoded Dam-like methyltransferase M.phi Ch1-I methylates adenine residues and complements dam mutants in the low salt environment of Escherichia coli
U. Baranyi et al., The archaeal halophilic virus-encoded Dam-like methyltransferase M.phi Ch1-I methylates adenine residues and complements dam mutants in the low salt environment of Escherichia coli, MOL MICROB, 35(5), 2000, pp. 1168-1179
The genome of the archaeal virus phi Ch1, infecting Natrialba magadii (form
erly Natronobacterium magadii), is composed of 58.5 kbp linear ds DNA. Viru
s particles contain several RNA species in sizes of 100-800 nucleotides. A
fraction of phi Ch1 genomes is modified within 5'-GATC-3' and related seque
nces, as determined by various restriction enzyme digestion analyses. High
performance liquid chromatography revealed a fifth base, in addition to the
four nucleosides, which was identified as N-6-methyladenosine. Genetic ana
lyses and subsequent sequencing led to the identification of a DNA (N-6-ade
nine) methyltransferase (mtase) gene. The protein product was designated M.
phi Ch1-I. By the localization of the most conserved motifs (a DPPY motif o
ccurring before FxGxG), the enzyme was placed within the beta-subgroup of t
he (N-6-adenine) methyltransferase class. The mtase gene of phi Ch1 was cla
ssified as a 'late' gene, as determined by measuring the kinetics of mRNA a
nd protein expression in N. magadii during the lytic cycle of phi Ch1. Afte
r infection of cells, M.phi Ch1-I mRNA and protein could be detected in low
er amounts than in the situation of virus induction from lysogenic cells. C
onsequently, only about 5% of the phi Ch1 progeny genomes after infection o
f N. magadii carry the M.phi Ch1-I methylation in contrast to 50% of virus
genomes generated by induction of phi Ch1-lysogenic N. magadii cells. Heter
ologous expression of the mtase from a halophile with 3 M cytoplasmic salt
concentration showed an unexpected feature: the protein was active in the l
ow environment of Escherichia coli and was able to methylate DNA in vivo. I
nterestingly, it seemed to exhibit a higher sequence specificity in E. coli
that resulted in adenine methylation exclusively in the sequence 5'-GATC-3
'. Additionally, expression of M.phi Ch1-I in dam(-) E. coli cells led to a
complete substitution of the function of M.Dam in DNA mismatch repair.