Characterization of short sequence repeats of Type III Restriction Modification Enzyme MboIII

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.