Molecular characterization of hsp20, encoding a small heat shock protein of Bifidobacterium breve UCC2003

Small heat shock proteins (sHSPs) are members of a diverse family of stress proteins that are important in cells to protect proteins under stressful conditions. Genome analysis of Bifidobacterium breve UCC2003 revealed a single sHSP-encoding gene, which was classified as a hsp20 gene by comparative analyses. Genomic surveillance of available genome sequences indicated that hsp20 homologs are not widely distributed in bacteria. In members of the genus Bifidobacterium, this gene appears to be present in only 7 of the 30 currently described species. Moreover, phylogenetic analysis using all available bacterial and eukaryotic sHSP sequences revealed a close relationship between bifidobacterial HSP20 and the class B sHSPs found in members of the division Firmicutes. The results of this comparative analysis and variation in codon usage content suggest that hsp20 was acquired by certain bifidobacteria through horizontal gene transfer. Analysis by slot blot, Northern blot, and primer extension experiments showed that transcription of hsp20 is strongly induced in response to severe heat shock regimens and by osmotic shock.