We have sequenced 200 to 240 bases of the matrix (M) glycoprotein gene of 23 strains of infectious bronchitis virus (IBV) representing the A (D207), B (D3896), C (D3128), D (D212), Massachusetts (Mass), UK11 and UK12 serotypes. The bases examined code for the external, hydrophilic region and the first membrane-embedded hydrophobic region of M, both regions comprising approximately 20 amino acids. As predicted from protein studies the A/D and B/C serotypes had two and one potential glycosylation sites respectively. This variation appeared to derive from a combination of base substitutions and deletions/insertions. The glycosylation sequence Asn-Cys-Thr was highly conserved. Overall, the exposed part of M exhibited a fourfold greater extent of amino acid variation than did the membrane-embedded sequence. The transcription-associated homology region sequence (CUUAACAA) in the 5′ intergenic region was identical in all strains but there was considerable variation as to its location. The M gene of UK12 appeared to have evolved from a group A-like M gene by a two stage process involving a base substitution in the intergenic region which generated a new AUG translation start codon followed by deletion of the original AUG. Isolate UK11 closely resembled Mass strains in the intergenic region but was dissimilar from all strains in the protein coding region. The M sequences of serotypes B and C were identical and those of the A and D serotypes very similar. These results are discussed in relation to recent sequencing of part of the spike glycoprotein gene of some of these strains and the discovery of recombination of murine hepatitis coronavirus.


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