Transcriptional analysis of the gdhA gene in Streptococcus thermophilus

Aims: To study the transcriptional analysis of glutamate dehydrogenase gene, involved in the amino acid conversion to aroma compound in Streptococcus thermophilus. Methods and Results: Analysis of the gdhA gene nucleotide sequence of S. thermophilus CNRZ1066 revealed that the coding region is 1353 nucleotides long. The deduced amino acids sequence exhibits the putative GDH active site and some conserved domains characteristic of family I of hexameric GDHs. Phylogenetic analysis revealed that the gdh gene of S. thermophilus clustered with the orthologues of other streptococci such as Streptococcus mutans, Streptococcus agalactiae and Streptococcus infantarius. Studying the structural organization of the gdhA locus the amino acid similarity of GDHs was higher than 87%, but the locus organization was not conserved. A dominant transcript of approximately 1.4 kbp was revealed by Northern blot hybridization, suggesting that gdhA mRNA is monocystronic. Primer extension showed that transcription start point of gdhA was localized 43 bp upstream of the potential start codon (ATG). Conclusions: The gdhA represents a monocistronic operon highly conserved in phylogenetic-related bacteria. Significance and Impact of the Study: A deeper knowledge of gdh transcriptional mechanisms could lead to develop S. thermophilus industrial starter cultures with optimized aromatic properties.