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DNA methyltransferase genes of Bacillus subtilis phages: comparison of their nucleotide sequences
Abstract:
The phi 3T DNA methyltransferase (Mtase) and most of the SP beta Mtase genes have been sequenced. With the exception of their promoters, no difference was found between the phi 3T and SP beta Mtase genes which code for an enzyme with a Mr of 50 507, consisting of 443 amino acids (aa). Comparison of the deduced aa sequence of the phi 3T/SP beta type Mtase (target specificity: GGCC and GCNGC) with that of the previously established sequence of the SPR Mtase (Buhk et al., 1984) which has the target specificity GGCC and CCGG, reveals strong similarities between these two types of enzymes. There is, however, one striking difference: both the phi 3T/SP beta and the SPR enzymes contain at different positions inserts of 33 aa, which have no homology to each other. We suggest that the methylation specificity unique to each of the two types of Mtases (GCNGC in phi 3T/SP beta; CCGG in SPR) depends on these inserts, while the GGCC-specific modification potential common to all Mtases is determined by structures conserved in both types of enzymes. A DNA fragment of non-modifying phage Z, which shows homology to both flanks of the SPR Mtase gene, was also sequenced. This segment can be described as a derivative of SPR DNA, in which the Mtase gene and sequences at its 5' end have been deleted, with the deletion extending between two direct repeats of 25 bp.
Insights
Researchers sequenced DNA methyltransferase (Mtase) genes from phi 3T and SP beta phages, finding similarities and unique 33-amino acid inserts. These inserts likely determine the distinct methylation specificities of these DNA methyltransferases.
Area of Science:
- Molecular Biology
- Genetics
- Enzymology
Background:
- DNA methyltransferases (Mtases) are crucial enzymes in DNA modification.
- Previous studies established the sequence and specificity of the SPR Mtase.
Purpose of the Study:
- To sequence and compare DNA methyltransferase (Mtase) genes from phi 3T and SP beta phages.
- To investigate the structural basis for methylation specificity differences between Mtase types.
Main Methods:
- Gene sequencing of phi 3T and SP beta DNA methyltransferase (Mtase) genes.
- Amino acid sequence comparison between phi 3T/SP beta Mtase and SPR Mtase.
- Analysis of unique sequence inserts and conserved regions.
Main Results:
- Phi 3T and SP beta Mtase genes share high sequence homology, differing mainly in their promoter regions.
- Both phi 3T/SP beta and SPR Mtases exhibit GGCC specificity, but differ in additional specificities (GCNGC for phi 3T/SP beta; CCGG for SPR).
- Unique 33-amino acid inserts were identified in phi 3T/SP beta and SPR Mtases, lacking homology and located at different positions.
Conclusions:
- The unique 33-amino acid inserts in Mtases are proposed to determine their distinct methylation specificities.
- Conserved structural elements likely account for the common GGCC methylation specificity across different Mtase types.
- A derivative DNA fragment from phage Z, lacking the Mtase gene, was also sequenced, showing homology to SPR Mtase gene flanks.