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ThegalM (mutarotase) gene of Streptococcus thermophilus F410: Partial sequencing, transcriptional analysis, relationship to downstream genes, and possible roles in explaining the galactose phenotypes of S. thermophilus strains
Abstract
Streptococcus thermophilus utilizes a lactose permease and the $\beta$-galactosidase enzyme to transport and hydrolyze lactose inside the cell. Most wild-type strains of S. thermophilus are unable to ferment lactose completely and release galactose into the medium. The strong repression of the galK gene was thought to be the primary reason for the galactose negative (Gal-) phenotype. The galK gene from S. thermophilus F410 has been previously cloned and sequenced. The chromosomal DNA region 5$\sp\prime$ to the galK gene was amplified by the use of PCR. A 0.7 kb PCR product was cloned and sequenced. Sequence analysis of the new PCR sequence revealed an open reading frame (ORF) that extended 174 bases past the transcriptional start site of the galK gene. In addition, the correct start codon for the galK gene was found to be located two bases downstream from the stop codon for this ORF. The peptide translated from this partial open reading frame had high similarity ($>$54%) to the galM gene product from Escherichia coli and other bacteria. Transcriptional analysis of the galM and the galK genes revealed that the galK gene is transcribed alone and as a polycistronic message that includes the galM and galK coding regions, with both transcripts being under catabolite repression by glucose. While the galK gene appears to be present in all strains of S. thermophilus, the galM gene was only detected in Gal$\sp+$ strains using Southern blot analysis. This may prove to be very important, since it has been reported that the mutarotase enzyme is required for the simultaneous metabolism of galactose with glucose from intracellular lactose hydrolysis in E. coli. Transcriptional and computer analysis of the DNA region immediately downstream of the galK gene indicated the presence of a putative galactose permease, one or more putative regulatory proteins, and a putative oligoendopeptidase gene.
Subject Area
Molecular biology
Recommended Citation
Zirnstein, Gerald Wayne, "ThegalM (mutarotase) gene of Streptococcus thermophilus F410: Partial sequencing, transcriptional analysis, relationship to downstream genes, and possible roles in explaining the galactose phenotypes of S. thermophilus strains" (1996). ETD collection for University of Nebraska-Lincoln. AAI9712536.
https://digitalcommons.unl.edu/dissertations/AAI9712536