Conserved serine-rich sequences in xylanase and cellulase from Pseudomonas fluorescens subspecies cellulosa: internal signal sequence and unusual protein processing

Abstract

The complete nucleotide sequence of the xynA gene coding for a xylanase (XYLA) expressed by Pseudomonas fluorescens subspecies cellulosa, has been determined. The structural gene consists of an open reading frame of 1833 bp followed by a TAA stop codon. Confirmation of the nucleotide sequence was obtained by comparing the predicted amino acid sequence with that derived by N-terminal analysis of purified forms of the xylanase. The signal peptide present at the N terminus of mature XYLA closely resembles signal peptides of other secreted proteins. Truncated forms of the xylanase gene, in which the sequence encoding the N-terminal signal peptide had been deleted, still expressed coli. XYLA contains domains which are homologous to an endoglucanase expressed by the same organism. These structures include serine-rich sequences. Bal31 deletions of xynA revealed the extent to which these conserved sequences, in XYLA, were essential for xylanase activity. Downstream of the TAA stop codon is a G + C-rich region of dyad symmetry (delta G = 24 kcal) characteristic of E. coli Rho-independent transcription terminators.