Abstract:
Short Sequence Repeats (SSR) are a means of achieving phase variation, i.e.
turning on/off of genes, in bacteria. Phase variation is known to be important for
survival of host-adapted pathogen. However, the effect of SSR on the activity of the
gene product is not clear. Type III enzymes are one of four types of Restriction-
Modification (RM) enzymes found in bacteria which protects them against foreign
DNA invasion. Many Type III enzymes have Short Sequence Repeats (SSR), which
have high propensity for phase variation. For example, the Type III enzyme MboIII
from the pathogen Mycoplasma bovis has 9 AG repeats. A three-dimensional
homology model of the enzyme revealed presence of repeat at the interface of the
three-subunit protein complex of MboIII. Sequence analysis revealed that the
position of the repeats is conserved amongst a number of Type III RM identified in as
many as 10 genera. Furthermore, we compared the nuclease and methyltransferase
activities of MboIII with and without repeats. Nuclease and methylation assays
suggested that MboIII with repeats cleaved and methylated the DNA appreciably
better than without repeat. This is a small step towards understanding the plastic
nature of Type III enzymes and dual nature of the mod gene. Detailed experiments
are required to be carried out to better understand the molecular effect of SSR on
enzyme structure and function.
