Characterization of the fructosyltransferase gene of Actinomyces naeslundii WVU45

Oral actinomycetes produce fructosyltransferase (FTF) enzymes which convert sucrose into polymers of D-fructose, known as levans, and these polymers are thought to contribute to the persistence and virulence of the organisms. A gene encoding FTF was isolated from Actinomyces naeslundii WVU45; the deduced amino acid sequence showed significant similarity to known levansucrases of gram-negative environmental isolates but was less similar to FTFs from gram-positive bacteria. A transcriptional start site was mapped by primer extension 70 bp 5' from the putative start codon, Promoter fusions to a chloramphenicol acetyltransferase gene were used to confirm that there was a functional promoter driving ftf expression and to show that sequences located 86 to 218 bp upstream of the transcription initiation site were required for optimal ftf expression. Quantitative slot blot analysis against total RNA from cells grown on different sugars or from different growth phases revealed that ftf was constitutively transcribed. Thus, the A. naeslundii FTF is more similar in primary sequence and the regulation of expression to levansucrases of gram-negative bacteria than gram-positive bacteria.