The chitinase gene cluster of the marine bacterium Pseudoalteromonas sp. S91, chiABC, which produces the major chitinases of this sp., was transcribed as an operon and from each individual gene. chiA, chiB and chiC were found to possess multiple transcriptional start points (TSPs), the use of which was determined by the nutrient regime used for S91 growth. In minimal medium containing glutamate, chiA, chiB and chiC each used 3, 1 and 1 TSP, respectively. Upon the addition of the chitin monomer N-acetylglucosamine, the number of chiA TSPs was unaffected. However, chiB used an additional 4 TSPs, and chiC used four new TSPs excluding the TSP used in glutamate only. In addition, the cluster was transcribed as an operon from TSP A1 of chiA. All TSPs were potentially associated with either a sigma(70)- or sigma(54)-dependent promoter. Under the growth conditions used, no TSPs were detected for chiB or chiC in S91CX, a chiA transposon mutant. The transcription of the S91 chiABC gene cluster produced at least four polycistronic mRNAs. In addition, the occurrence of operon transcription of chiABC, and identification of an additional 12 putative TSPs within the gene cluster, gave an indication that each gene appeared to be transcribed from more than one promoter region upstream of each in-frame translation start codon. Questions arose regarding the reason for this complexity of transcription within the gene cluster, leading to a re-evaluation of the Chi protein domains. By bioinformatic review, ChiA, ChiB and ChiC were found to potentially possess additional putative domains.