Two genes, COL4A3 and COL4A4 coding for the human α3(IV) and α4(IV) collagen chains are arranged head-to-head on chromosome 2q36

We first isolated and characterized genomic DNA fragments that cover the 5' flanking sequences of COL4A3 and COL4A4 encoding the human basement membrane α3(IV) and α4(IV) collagen chains, respectively. Nucleotide sequence analysis indicated that the two genes are arranged head-to-head. To determine transcription start site for COL4A4 gene, we performed RACE and RNase protection assays, indicating that there are two alternative transcripts presumably derived from two different promoters. Interestingly, one transcription start site (from exon 1') of COL4A4 is only 5 bp away from the reported transcription start site of COL4A3, whereas the other transcript (from exon 1) starts 373 nucleotides downstream from the first one, generating the two kinds of transcripts that differ in the 5' UTR regions. Expression of these two transcripts appears tissue-specific; exon 1 transcript was expressed predominantly in epithelial cells, while exon 1' transcript showed rather ubiquitous and low expression. The nucleotide sequence of the promoter region is composed of dense CpG dinucleotides, GC boxes, CTC boxes and a CCAAT box but no TATA box. These results provide information to delineate the promoter activity for the tissue-specific expression of the six type IV collagen genes and basement membrane assembly in different tissues and organs.