Isolation and characterization of a cellulase gene family member expressed during avocado fruit ripening

We present in this paper the structural analysis of two members of a small cellulase gene family, designated cel1 and cel2, from avocado. These genes were isolated by screening a lambda EMBL3 genomic library with a ripening-induced cellulase cDNA. Restriction endonuclease and Southern blot analyses showed that the cel1 gene is highly homologous to the cellulase cDNA and thus represents a ripening-related cellulase gene. The other cellulase gene, cel2, is closely related to cel1, but is divergent at its 5' end. The nucleotide sequence of a 5 kb region encompassing the cel1 gene was determined. Four previously characterized cellulase cDNAs from ripe fruit are identical to the eight exons of the cel1 gene. RNase protection and primer extension analyses were used to define the transcription start site of cel1 and to quantitate cel1 transcripts in ripening fruit. The cel1 mRNA was present at a low level in unripe fruit and increased 37-fold during ripening. Partial DNA sequence analysis of cel2 and comparison to the cel1 sequence revealed a high degree of similarity both at the DNA and deduced amino acid sequence levels. No characterized cellulase cDNAs derived from ripe fruit represent cel2 transcripts. These data suggest that the cel1 gene is responsible for a major portion, if not all, of the cellulase transcripts in ripe fruit. The DNA sequence of 1.4 kb of 5' flanking DNA of the cel1 gene was compared to the upstream sequence of other ethylene-regulated genes. Several interesting upstream sequence motifs were identified and are discussed.