Temperate phages are common and prophages are abundant residents of sequenced bacterial genomes. Mycobacteriophages are viruses infecting mycobacterial hosts including Mycobacterium tuberculosis and Mycobacterium smegmatis, encompass substantial genetic diversity, and are commonly temperate. Characterization of ten Cluster N temperate mycobacteriophages reveals at least five distinct prophage-expressed viral defense systems that interfere with infection of lytic and temperate phages that are either closely-related (homotypic defense) or unrelated (heterotypic defense). Target specificity is unpredictable, ranging from a single target phage to one-third of those tested. The defense systems include a single-subunit restriction system, a heterotypic exclusion system, and a predicted (p)ppGpp synthetase, which blocks lytic phage growth, promotes bacterial survival, and enables efficient lysogeny. The predicted (p)ppGpp synthetase coded by the Phrann prophage defends against phage Tweety infection, but Tweety codes for a tetrapeptide repeat protein, gp54, that acts as a highly effective counter-defense system. Prophage-mediated viral defense offers an efficient mechanism for bacterial success in host-virus dynamics, and counter-defense promotes phage co-evolution.

The molecular defect in a patient with a moderately severe form of osteogenesis imperfecta was characterized by nuclease S1 mapping. Single-stranded 5' and 3' endlabeled DNA probes coding for 80% of the carboxyl-propeptide of the proa2(I) collagen gene were hybridized to mRNA isolated from cultured fibroblasts of the patient and his parents. Nuclease S1 digestion revealed a homozygous mutation in the patient and a heterozygous pattern in the consanguineous parents. As a result of the defect in the gene, none of the proa2(I) chains synthesized by the patient's fibroblasts were incorporated into a type I procollagen heterotrimer consisting of two proal(I) chains and one proa2(I) chain. Cultured skin fibroblasts from the patient have previously been shown to secrete only proial(I) trimers. As shown here, fibroblasts from both parents, who do not have osteogenesis imperfecta, secrete both proal(I) trimers and normal type I procollagen. A further observation was that synthesis of proa2(I) chains was decreased in fibroblasts from the patient and his parents. The decrease in the synthesis of proa2(I) chains is not caused by decreased transcription of the proa2(I) collagen alleles, since the proal(I)/proa2(I) mRNA ratios were normal in the patient and his parents.

We have located the exon coding for the start site of transcription of the human pro alpha 2(I) collagen gene. Comparison with the homologous region of other fibrillar collagen genes has confirmed the existence of a consensus sequence (CATGTCTA-n-TAGACATG) capable of forming a hairpin secondary structure possibly involved in the regulation of collagen biosynthesis. Sequence comparison of the chromosomal regions at the 5' end of the pro alpha 1(I) and pro alpha 2(I) collagen genes failed to identify unique DNA elements potentially mediating common regulatory signals. Sequencing of four exons coding for the N-terminal propeptide has determined most of its structure and it has implied the existence of smaller coding units similar to the 11 and 18 bp exons originally described in the avian gene.

In order to elucidate some of the mechanisms leading to the pathological expression of the human fibrillar collagens, as well as to understand the evolution of these loci, specific cDNA and genomic clones have been isolated. The primary structure of the COOH-terminal propeptide of the four collagen chains and either part or the entire exon/intron arrangement of the genes have been determined. Interspecies and pairwise comparison revealed that the four loci have evolved at slightly different rates, maintaining, however, remarkably similar exon/intron arrangement. The fibrillar genes, albeit sharing the same elaborate structure, exhibit different sizes that correlate with the average length of their intron sequences, possibly because of their different chromosomal origin.

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