Ira Simet

Two areas of investigation, both centered on enzyme-catalyzed biopolymerizations, occupy our research efforts:

1. Amylose synthesis from sucrose by oral bacteria. Dental caries (tooth decay) is the most prevalent disease of the calcified tissues of the mouth, affecting 99% of the U.S. population. This disease is associated with the presence of dental plaque, a deposit of bacterial polysaccharides and proteins that anchors bacteria to tooth surfaces and promotes accumulation of acidic metabolites from those bacteria. Plaque polysaccharides are synthesized by oral bacteria through polymerization of the glucose portions of sucrose. The enzymes that carry out these polymerizations, the dextransucrases, are unique to bacteria and therefore represent attractive targets for anti-decay therapies.
   
   A strain of nonpathogenic oral Neisseria assembles unbranched amylose-like glucose polymers under conditions of sparse bacterial population and high levels of available sucrose. As amylose is a very minor component of established plaque, these bacteria are more likely to be involved in initial plaque deposition than in maintenance of existing plaque. We currently are isolating and purifying the dextransucrase responsible for the synthesis of amylose-like polymers. We also have discovered that these bacteria produce different highly-branched glucose polymers when colonization density is high and/or sucrose availability decreases. We are pursuing this interesting observation to learn more about the mechanisms for control of bacterial dextransucrases.
    
2. Multiple forms of DNA polymerase alpha in embryonic chicken brain. DNA polymerases, the enzymes that replicate DNA, are crucial components of the machinery of cell growth. In eukaryotic cells, several different DNA polymerases have been characterized, but their roles in DNA replication remain unclear. DNA polymerase alpha appears to be the chief duplicating enzyme, while other DNA polymerases are more likely to be involved in DNA repair or to have roles in minor events during DNA duplication. Clarification of these functions is important to complete understanding of cell growth in both normal and abnormal (cancerous) cells.
   
   Our laboratory has resolved and partially purified three distinct DNA polymerizing enzymes (DNAP alpha-1, DNAP alpha-2, and DNAP alpha-3) that have properties attributed to DNA polymerase alpha. Kinetic analysis has shown that all three enzymes share a common catalytic protein (or proteins) with a characteristic affinity for deoxynucleotides. However, the enzymes differ in their utilization of various DNA template-primer substrates, which may reflect the presence or absence of DNA-binding proteins associated with the common catalytic protein(s). We currently are evaluating the protein compositions of the three enzymes to locate potential DNA-binding subunits. We also are testing the three forms of DNA polymerase alpha for priming and/or exonucleolytic activities, which have been reported for other eukaryotic DNA polymerases.