DNA sequence of the Serratia marcescens lipoprotein gene

TheSerratia marcescens gene for the outer membrane lipoprotein (lpp ) was cloned in λ phage vector Charon 14. The recombinant phage was very unstable, and thelpp gene with a 300-base-pair deletion at the transcription termination site was further cloned in pBR322. The DNA sequence of 834 base pairs encompassing thelpp gene was determined and compared with that of theEscherichia coli lpp gene. The sequence comparisons exhibit several unique features. (i ) The promoter region is highly conserved (84% homology) and has an extremely high A+T content (78%) as inE. coli (80%). (ii ) The 5′ nontranslated region of the lipoprotein mRNA is also highly conserved (95% homology). (iii ) In the DNA sequence corresponding to the signal peptide of this secretory protein, there are three drastic changes, including addition of one base pair and deletion of four base pairs inS. marcescens as compared toE. coli . The resultant alterations in the amino acid sequence, however, do not change the basic properties of the signal peptide, which are assumed to be essential for its function in the secretory mechanism. (iv ) The DNA sequence from the amino terminus to the 51st residue of the mature lipoprotein is highly conserved (95% homology) and there is no amino acid substitution. (v ) The DNA sequence corresponding to the seven amino acid residues at the carboxyl terminus has only 42% homology, resulting in four amino acid substitutions. (vi ) Within the section of 40 base pairs beginning with the termination codon (UAA) and ending immediately before the oligo(T) transcription termination site in theE. coli lpp gene, there is about 60% homology. However, after this section, there is no obvious homology between the two sequences, probably because of a deletion of 300 base pairs at this region. (vii ) Seven stable stem-and-loop structures could be formed in the mRNA region. (viii ) Alterations in the third position of codons used in thelpp gene suggest that the gene has evolved somewhat differently from other genes inS. marcescens .