Nucleotide sequence of a second alpha giardin gene and molecular analysis of the alpha giardin genes and transcripts in Giardia lamblia.

The giardins are a group of proteins with relative molecular masses (Mrs) between 29,000 and 38,000 that are specific to the ventral disk of the intestinal protozoan parasite Giardia lamblia. We previously have characterized alpha-giardin, renamed here alpha-1-giardin, as a novel 33-kDa protein located on the edges of the disk microribbons. Southern blot analysis of G. lamblia genomic DNA, followed by cloning and sequencing, revealed the existence of a related gene that we have called alpha-2-giardin. Sequence comparison of the alpha-giardin genes reveals 81% identity at the nucleotide level and 77% at the predicted amino acid level. The predicted alpha-giardins have similar Mrs of approximately 33,900 and are very rich in alpha-helix conformations. Each gene is present in single copy and, like many other known Giardia coding sequences, exhibits a strong preference for cytidine and guanosine in the third base position of each codon. Chromosome hybridization analysis indicates that both genes are either on the same chromosome or on chromosomes with similar mobility. Experiments utilizing primer extension and RNA sequencing provide evidence that both genes are transcribed. The stable transcripts have extremely short leader regions of only 3 nucleotides, and the downstream sequence of the alpha-2-giardin gene reveals that the sequence AGTPuAA remains a consistent element within G. lamblia protein-encoding genes.