CLONING AND MOLECULAR CHARACTERIZATION OF THE DNA-LIGASE GENE (LIG) FROM ZYMOMONAS-MOBILIS

The Zymomonas mobilis lig gene that encodes DNA ligase was cloned from a cosmid library and identified by genetic complementation of a conditional-lethal Escherichia coli DNA ligase mutant. Nucleotide sequence analysis of the Z. mobilis lig region indicated that the gene is 2196 bp long, encoding a protein with a deduced molecular mass of 82089. The primary amino acid sequence of the Z. mobilis ligase is 48% identical to the E. coli enzyme. Two genes located upstream of lig were identified as tgt, encoding tRNA guanine transglycosylase and uvrB, encoding the beta-subunit of excision endonuclease. Computer searches did not reveal any transcriptional terminators in the 46-bp tgt-lig intergenic region, suggesting that lig may be cotranscribed with one or more upstream genes. Weak expression of lig is explained in part by frequent use of codons that are known to be rarely used in the highly expressed glycolytic gene set.