Determination of DNA Transfer Frequency, Related to the Presence of comX Gene, in Natural Streptococcus thermophilus Isolates

Activity of comX, alternative sigma factor, is crucial in the expression of the genes required for conjugation takes place in Streptococci and hence it is related to the frequency of competency. Determination of these genes in natural isolates, therefore, is a pivotal issue to understand the conjugation capacity of those microorganisms. In the current study, 106 Streptococcus thermophilus isolates are investigated for the presence of comX gene. Total 46 of these isolates are found to be comX positive using polymerase chain reaction (PCR) and nucleotide sequencing data of them were obtained. The plasmid pNZ276 and catalase tests were used to investigate the conjugation frequency of competent cells. Conjugation characteristics of the isolates BioSt-7, 16, 19, 23, 24, 25, 29, 30, 36, 37, 42 and 46 were studied and conjugation frequency of the isolates BioSt-19, 24, 25,29 and 42 are found to be over 40% and moreover a notable relationship between conjugation ratio and comX gene is determined. Substitution of isoleucine (I) for asparagine (N) in 72nd amino acid position of comX protein increased the conjugation frequency of the isolates. The findings of current study suggest that, some mutations occurred on comX gene may affect the frequency of competency and conjugated characteristics of streptococcal isolates