Provided are nucleic acids that include a promoter, where the promoter is operable in a Bacteroides cell and is operably linked to a heterologous nucleotide sequence of interest. Also provided are nucleic acids that include a promoter (operable in a prokaryotic cell such as a Bacteroides cell) operably linked to a sequence encoding a synthetic ribosomal binding site (RBS). Also provided are fusion proteins (and nucleic acids encoding them) in which a secreted Bacteroides polypeptide is fused to a heterologous polypeptide of interest. Also provided are prokaryotic cells (e.g., E. coli, a Bacteroides cell, and the like) that include one more nucleic acids such as those described above. Also provided are methods of expression in a prokaryotic cell, methods of detectably labeling a Bacteroides cell in an animal's gut, and methods of delivering a protein to an individual's gut.